Author name code: mikic
ADS astronomy entries on 2022-09-14
author:"Mikic, Zoran"
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Title: The role of asymmetries in coronal rain formation during
thermal non-equilibrium cycles
Authors: Pelouze, Gabriel; Auchère, Frédéric; Bocchialini, Karine;
Froment, Clara; Mikić, Zoran; Soubrié, Elie; Voyeux, Alfred
Bibcode: 2022A&A...658A..71P
Altcode: 2021arXiv211009975P
Context. Thermal non-equilibrium (TNE) produces several observables
that can be used to constrain the spatial and temporal distribution
of solar coronal heating. Its manifestations include prominence
formation, coronal rain, and long-period intensity pulsations in
coronal loops. The recent observation of abundant periodic coronal rain
associated with intensity pulsations allowed for these two phenomena
to be unified as the result of TNE condensation and evaporation
cycles. On the other hand, many observed intensity pulsation events
show little to no coronal rain formation.
Aims: Our goal is
to understand why some TNE cycles produce such abundant coronal
rain, while others produce little to no rain.
Methods:
We reconstructed the geometry of the periodic coronal rain event,
using images from the Extreme Ultraviolet Imager (EUVI) onboard the
Solar Terrestrial Relations Observatory (STEREO), and magnetograms
from the Helioseismic and Magnetic Imager (HMI). We then performed 1D
hydrodynamic simulations of this event for different heating parameters
and variations of the loop geometry (9000 simulations in total). We
compared the resulting behaviour to simulations of TNE cycles that do
not produce coronal rain.
Results: Our simulations show that
both prominences and TNE cycles (with and without coronal rain) can
form within the same magnetic structure. We show that the formation
of coronal rain during TNE cycles depends on the asymmetry of the
loop and of the heating. Asymmetric loops are overall less likely
to produce coronal rain, regardless of the heating. In symmetric
loops, coronal rain forms when the heating is also symmetric. In
asymmetric loops, rain forms only when the heating compensates for
the asymmetry. Movie associated to Fig. 5 is available at https://www.aanda.org
Title: Necessary Conditions for a Hot Quiet Sun Atmosphere:
Chromospheric Flares and Low Corona Twisted Flux Rope Eruptions
Authors: Amari, Tahar; Luciani, Jean-Francois; Aly, Jean-Jacques;
Canou, Aurelien; Mikic, Zoran; Velli, Marco
Bibcode: 2021AGUFMSH12B..05A
Altcode:
The issue of relevant scales involved in the heating of the solar
atmosphere is an important one. Since the temperature already reaches 1
MK a few megameters above the photosphere, observations made by Parker
Solar Probe will be able to explore those at larger heights but only
indirectly at those lower heights, where small scale coupling between
sub-photospheric, chromospheric and coronal structure and dynamics
occurs. While Solar Orbiter will be able to bring such observations,
modeling appears a complementary interesting approach to interpret
those observations Taking a sub-surface dynamo and a sharp realistic
VAL- like scale profile from photosphere to corona, with a fixed
temperature profile in time, we investigate the necessary conditions
implied on the structures and dynamics of the atmosphere to keep this
thermal structuration, as well as their implication in the energy
budget of the atmosphere. Under those hypothesis we show that :i)
the transverse photospheric field below 100km plays a major role;
ii) an associated scale of one megameter activity naturally results
to produce a zone above the photosphere with high confined electric
currents, which then expands into the chromosphere and releases energy(4
500 W/m2) through small-scale eruptions driving sonic motions; iii)
meso scale structuration, leads to the formation of larger coherent
twisted flux ropes, and associated eruptive like activity in a way
similar to large scale eruptive phenomena, as result of cancellation,
emergence, and convergence motions. Finally a wave dynamics is also
naturally driven in core corona associated to above 300 W/m2.
Title: Reproducibility package for running the DIFFUSE test cases from
"Can Fortran's `do concurrent` replace directives for accelerated
computing"
Authors: Mikic, Zoran; Caplan, Ronald M.; Linker, Jon A.; Stulajter,
Miko
Bibcode: 2021zndo...5253520M
Altcode:
Reproducibility package for the paper "Can Fortran's `do concurrent`
replace directives for accelerated computing?". The package contains
the three singularity containers (for gfortran, nvfortran, and ifort)
used, as well as all code versions, compiler options, and test
cases. The package requires minimal customization (only specifying
hardware-specific compiler options) of the main script, which can then
be used to automatically run either all, or a subset, of runs from
the paper. See the documentation in the package for more details. A
reference solution is also provided for validation. Note that runs
using GPU-acceleration require having an NVIDIA GPU with compatible
drivers installed on the system.
Title: Variations in Finite-difference Potential Fields
Authors: Caplan, Ronald M.; Downs, Cooper; Linker, Jon A.; Mikic, Zoran
Bibcode: 2021ApJ...915...44C
Altcode: 2021arXiv210205618C
The potential field (PF) solution of the solar corona is a vital
modeling tool for a wide range of applications, including minimum
energy estimates, coronal magnetic field modeling, and empirical solar
wind solutions. Given its popularity, it is important to understand
how choices made in computing a PF may influence key properties
of the solution. Here we study PF solutions for the global coronal
magnetic field on 2012 June 13, computed with our high-performance
finite-difference code POT3D. Solutions are analyzed for their global
properties and locally around NOAA AR 11504, using the net open flux,
open-field boundaries, total magnetic energy, and magnetic structure
as metrics. We explore how PF solutions depend on (1) the data source,
type, and processing of the inner boundary conditions; (2) the choice
of the outer boundary condition height and type; and (3) the numerical
resolution and spatial scale of information at the lower boundary. We
discuss the various qualitative and quantitative differences that
naturally arise by using different maps as input, and we illustrate
how coronal morphology and open flux depend most strongly on the outer
boundary condition. We also show how large-scale morphologies and the
open magnetic flux are remarkably insensitive to model resolution,
while the surface mapping and embedded magnetic complexity vary
considerably. This establishes important context for past, current,
and future applications of the PF for coronal and solar wind modeling.
Title: Slip-back Mapping as a Tracker of Topological Changes in
Evolving Magnetic Configurations
Authors: Lionello, R.; Titov, V. S.; Mikić, Z.; Linker, J. A.
Bibcode: 2020ApJ...891...14L
Altcode: 2019arXiv190501384L
The topology of the coronal magnetic field has a strong impact on
the properties of the solar corona and presumably on the origin of
the slow solar wind. To advance our understanding of this impact,
we revisit the concept of so-called slip-back mapping and adapt it
to determine open, closed, and disconnected flux systems that are
formed in the solar corona by magnetic reconnection during a given
time interval. In particular, the method we developed allows us to
describe magnetic flux transfer between open and closed flux regions
via so-called interchange reconnection with an unprecedented level of
detail. We illustrate the application of this method to the analysis
of the global MHD evolution of the solar corona driven by idealized
differential rotation of the photospheric plasma.
Title: Exploring Plasma Heating in the Current Sheet Region in a
Three-dimensional Coronal Mass Ejection Simulation
Authors: Reeves, Katharine K.; Török, Tibor; Mikić, Zoran; Linker,
Jon; Murphy, Nicholas A.
Bibcode: 2019ApJ...887..103R
Altcode: 2019arXiv191005386R
We simulate a coronal mass ejection using a three-dimensional
magnetohydrodynamic code that includes coronal heating, thermal
conduction, and radiative cooling in the energy equation. The magnetic
flux distribution at 1 R s is produced by a localized
subsurface dipole superimposed on a global dipole field, mimicking
the presence of an active region within the global corona. Transverse
electric fields are applied near the polarity inversion line to
introduce a transverse magnetic field, followed by the imposition of
a converging flow to form and destabilize a flux rope, producing an
eruption. We examine the quantities responsible for plasma heating and
cooling during the eruption, including thermal conduction, radiation,
adiabatic effects, coronal heating, and ohmic heating. We find that
ohmic heating is an important contributor to hot temperatures in the
current sheet region early in the eruption, but in the late phase,
adiabatic compression plays an important role in heating the plasma
there. Thermal conduction also plays an important role in the transport
of thermal energy away from the current sheet region throughout the
reconnection process, producing a “thermal halo” and widening the
region of high temperatures. We simulate emission from solar telescopes
for this eruption and find that there is evidence for emission from
heated plasma above the flare loops late in the eruption, when the
adiabatic heating is the dominant heating term. These results provide an
explanation for hot supra-arcade plasma sheets that are often observed
in X-rays and extreme ultraviolet wavelengths during the decay phase
of large flares.
Title: Solar Eruptions Triggered by Flux Emergence
Authors: Torok, T.; Linton, M.; Leake, J. E.; Mikic, Z.; Titov, V. S.;
Lionello, R.
Bibcode: 2019AGUFMSH33B3390T
Altcode:
Observations have shown a clear association of prominence eruptions
and CMEs with the emergence of magnetic flux close to, or within,
filament channels. It has been suggested that reconnection triggered
by the emergence destroys the force balance between the magnetic
field in the filament channel and its ambient field, causing the
former to erupt. Magnetohydrodynamic (MHD) numerical simulations
support this scenario for two-dimensional (2D) coronal flux-rope
configurations. However, such simulations do not take into account 3D
effects such as the anchoring of the flux rope in the dense photosphere
or the occurrence of 3D MHD instabilities. Here we present the first
3D MHD simulations of (boundary-driven) flux emergence in the vicinity
of a pre-existing coronal flux rope. We find that three processes
are important for the evolution of the system: (1) expansion or
contraction of the coronal field due to the intrusion of new flux,
(2) reconnection between the emerging and pre-existing flux systems,
and (3) repulsion or attraction of the respective current channels. We
vary the position and orientation of the emerging flux and investigate
under which conditions these processes can trigger an eruption.
Title: Tracking Topological Changes in MHD Simulations using
Slip-Back Mapping
Authors: Lionello, R.; Titov, V. S.; Linker, J.; Mikic, Z.
Bibcode: 2019AGUFMSH53B3379L
Altcode:
The topology of the coronal magnetic field produces a strong impact
on the properties of the solar corona and presumably on the origin
of the slow solar wind. To advance our understanding of this impact,
we revisit the concept of the so-called slip-back mapping (Titov et
al. 2009) and adapt it to determine open, closed, and disconnected flux
systems that are formed in the solar corona by magnetic reconnection
during a given time interval. In particular, the method we developed
allows us to describe the magnetic flux transfer between open and closed
flux regions via so-called interchange reconnection with unprecedented
level of details. We illustrate the application of this method to the
analysis of a global MHD evolution of the solar corona that is driven
by idealized differential rotation of the photospheric plasma.
Title: Validation of MHD Model Predictions of the Corona with LASCO-C2
Polarized Brightness Images
Authors: Lamy, Philippe; Floyd, Olivier; Mikić, Zoran; Riley, Pete
Bibcode: 2019SoPh..294..162L
Altcode:
Progress in our understanding of the solar corona requires that the
results of advanced magnetohydrodynamic models driven by measured
magnetic fields, and particularly the underlying heating models, be
thoroughly compared with coronal observations. The comparison has so far
mainly concerned the global morphology of the corona, synthetic images
calculated from the models being compared with observed images. We go
one step further by performing detailed quantitative comparisons between
the calculated polarized radiance p B using the three-dimensional
electron density produced by MHD models and well calibrated polarized
images obtained by the Large Angle Spectrometric Coronagraph LASCO-C2
coronagraph complemented by ground-based images when available from
the Mauna Loa Solar Observatory Mark IV and K-Cor instruments to extend
the comparison to the inner coronal region 1.0 - 2.5 R⊙,
which is inaccessible to C2. We take advantage of the high-resolution
and high-quality MHD predictions performed for several solar eclipses
(1 August 2008, 11 July 2010, 13 November 2012, and 21 August 2017) and
for the first perihelion passage of the Parker Solar Probe (5 November
2018) using two different three-dimensional MHD models relying on either
a thermodynamic or a wave-turbulence-driven methodologies to heat the
corona. Both models are generally able to match the observed structure
and photometry of the corona albeit with various degrees of fidelity for
which there is no obvious explanation. However, two limitations emerge,
the complexity of coronae of the maximum type and the time lapse between
the completion of the magnetograph measurements and the prediction.
Title: Can an Unobserved Concentration of Magnetic Flux Above the
Poles of the Sun Resolve the Open Flux Problem?
Authors: Riley, Pete; Linker, Jon A.; Mikic, Zoran; Caplan, Ronald M.;
Downs, Cooper; Thumm, Jean-Luc
Bibcode: 2019ApJ...884...18R
Altcode:
Global models of the extended solar corona, driven by observed
photospheric magnetic fields, generally cannot reproduce the amplitude
of the measured interplanetary magnetic field at 1 au (or elsewhere
in the heliosphere), often underestimating it by a factor of two or
more. Some modelers have attempted to resolve this “open flux”
problem by adjusting what they believe to be errors in the estimates
of the photospheric field values. Others have simply multiplied
interplanetary estimates by some correction factor to match 1 au
values. Here, we investigate whether this “missing” flux can be
explained by a source of largely unobserved, concentrated bundles
of flux in the photosphere at latitudes too high to be adequately
resolved by ground-based observatories or Earth-based spacecraft. Using
potential field source-surface and magnetohydrodynamic models, we
demonstrate that this additional polar flux can (at least partially)
resolve the open flux problem, without generating any new observational
discrepancies. For example, we show that model solutions without
this additional flux systematically produce streams lying at higher
hello-latitudes than is inferred from observations. More importantly,
adding this polar flux to the models does not substantially change the
location or size of computed coronal holes. The upcoming joint ESA/NASA
Solar Orbiter mission may be able to support or refute this idea.
Title: Bounding the Energy of Solar Eruptions
Authors: Linker, Jon A.; Downs, Cooper; Caplan, Ronald M.; Torok,
Tibor; Riley, Pete; Titov, Viacheslav; Lionello, Roberto; Mikic,
Zoran; Amari, Tahar
Bibcode: 2019AAS...23431704L
Altcode:
Major solar eruptions such as X-class flares and coronal mass ejections
(CMEs) are the fundamental source of solar energetic particles and
geomagnetic storms, and are thus key drivers of space weather at
Earth. The energy for solar eruptions is recognized to originate in
the solar magnetic field, and is believed to be stored as free magnetic
energy (energy above the potential field state) prior to eruption. Solar
active regions are the site of the most violent activity. Solar active
regions can store widely varying amounts of energy, so knowledge of
the free energy alone does not necessarily tell us when an eruption
is imminent. For estimates of the free energy to provide predictive
power, we must know how much energy a region can store - what is the
energy bound? In recent work, we have found that the energy of a
particular field, the partially open field (POF), can place a useful
bound on the energy of an eruption from real active regions, a much
tighter constraint than the energy of the fully open field. However,
in general, it is difficult to solve for the POF. In this presentation,
we discuss methods for approximating the energy of this field, and
show a comparison of the approximation for a case where the solution
is known. We discuss the implications for understanding and predicting
major solar eruptions. Research supported by NASA and AFOSR
Title: GPU Acceleration of an Established Solar MHD Code using OpenACC
Authors: Caplan, R. M.; Linker, J. A.; Mikić, Z.; Downs, C.; Török,
T.; Titov, V. S.
Bibcode: 2019JPhCS1225a2012C
Altcode: 2018arXiv181102605C
GPU accelerators have had a notable impact on high-performance
computing across many disciplines. They provide high performance with
low cost/power, and therefore have become a primary compute resource
on many of the largest supercomputers. Here, we implement multi-GPU
acceleration into our Solar MHD code (MAS) using OpenACC in a fully
portable, single-source manner. Our preliminary implementation is
focused on MAS running in a reduced physics “zero-beta” mode. While
valuable on its own, our main goal is to pave the way for a full
physics, thermodynamic MHD implementation. We describe the OpenACC
implementation methodology and challenges. “Time-to-solution”
performance results of a production-level flux rope eruption
simulation on multi-CPU and multi-GPU systems are shown. We find that
the GPU-accelerated MAS code has the ability to run “zero-beta”
simulations on a single multi-GPU server at speeds previously requiring
multiple CPU server-nodes of a supercomputer.
Title: Slip-Back Mapping as a Tracker of Topological Changes in
Evolving Magnetic Configurations
Authors: Titov, Viacheslav S.; Lionello, R.; Mikic, Z.; Linker, J. A.
Bibcode: 2019shin.confE.116T
Altcode:
The topology of the coronal magnetic field produces a strong impact on
the properties of the solar corona and presumably on the origin of the
slow solar wind. To advance our understanding of this impact, we revisit
the concept of the so-called slip-back mapping (Titov et al. 2009) and
adapt it for determining open, closed, and disconnected flux systems
that are formed in the solar corona by magnetic reconnection during a
given time interval. The method we have developed on this basis allows
us, in particular, to describe the magnetic flux transfer between
these systems via so-called interchange reconnection with unprecedented
level of details. The present implementation of the method relies on
the tracking of magnetic field lines through their moving footpoints at
both lower and upper spherical boundaries, which is not always possible
if there is a radial flow at the polarity inversion line. Nevertheless,
even with this restricted implementation, the proposed method enables us
to analyze a global MHD evolution of the solar corona that is driven by
idealized differential rotation of the photospheric plasma (Lionello
et al. 2005). To overcome the indicated restriction, we propose an
extension of the method that will allow investigating magnetic field
evolutions driven by more general boundary conditions. We anticipate
that the generalized method will be particularly useful for the analysis
of global MHD models combined with solar wind measurements from the
Solar Probe Plus and Solar Orbiter missions. Research supported
by NASA’s HSR, LWS, and HGI programs, NSF grant AGS-1560411, and
AFOSR contract FA9550-15-C-0001.
Title: Predicting the Structure of the Solar Corona and Inner
Heliosphere during Parker Solar Probe's First Perihelion Pass
Authors: Riley, Pete; Downs, Cooper; Linker, Jon A.; Mikic, Zoran;
Lionello, Roberto; Caplan, Ronald M.
Bibcode: 2019ApJ...874L..15R
Altcode: 2019arXiv190209673R
NASA’s Parker Solar Probe (PSP) spacecraft reached its first
perihelion of 35.7 solar radii on 2018 November 5. To aid in mission
planning, and in anticipation of the unprecedented measurements
to be returned, in late October, we developed a three-dimensional
magnetohydrodynamic (MHD) solution for the solar corona and inner
heliosphere, driven by the then available observations of the
Sun’s photospheric magnetic field. Our model incorporates a
wave-turbulence-driven model to heat the corona. Here, we present
our predictions for the structure of the solar corona and the likely
in situ measurements that PSP will be returning over the next few
months. We infer that, in the days prior to first encounter, PSP was
immersed in wind emanating from a well-established, positive-polarity
northern polar coronal hole. During the encounter, however, field
lines from the spacecraft mapped to a negative-polarity equatorial
coronal hole, within which it remained for the entire encounter, before
becoming magnetically connected to a positive-polarity equatorial
coronal hole. When the PSP data become available, these model results
can be used to assist in their calibration and interpretation, and,
additionally, provide a global context for interpreting the localized in
situ measurements. In particular, we can identify what types of solar
wind PSP encountered, what the underlying magnetic structure was, and
how complexities in the orbital trajectory can be interpreted within
a global, inertial frame. Ultimately, the measurements returned by
PSP can be used to constrain current theories for heating the solar
corona and accelerating the solar wind.
Title: Particle Radiation Sources, Propagation and Interactions
in Deep Space, at Earth, the Moon, Mars, and Beyond: Examples of
Radiation Interactions and Effects
Authors: Schwadron, Nathan A.; Cooper, John F.; Desai, Mihir; Downs,
Cooper; Gorby, Matt; Jordan, Andrew P.; Joyce, Colin J.; Kozarev,
Kamen; Linker, Jon A.; Mikíc, Zoran; Riley, Pete; Spence, Harlan E.;
Török, Tibor; Townsend, Lawrence W.; Wilson, Jody K.; Zeitlin, Cary
Bibcode: 2019sfsw.book..257S
Altcode:
No abstract at ADS
Title: Ion Charge States in a Time-Dependent Wave-Turbulence-Driven
Model of the Solar Wind
Authors: Lionello, Roberto; Downs, Cooper; Linker, Jon A.; Mikić,
Zoran; Raymond, John; Shen, Chengcai; Velli, Marco
Bibcode: 2019SoPh..294...13L
Altcode: 2019arXiv190103748L; 2018SoPh..294...13L
Ion fractional charge states, measured in situ in the heliosphere,
depend on the properties of the plasma in the inner corona. As the ions
travel outward in the solar wind and the electron density drops, the
charge states remain essentially unaltered or "frozen in". Thus they
can provide a powerful constraint on heating models of the corona and
acceleration of the solar wind. We have implemented non-equilibrium
ionization calculations into a 1D wave-turbulence-driven (WTD)
hydrodynamic solar wind model and compared modeled charge states
with the Ulysses 1994 - 1995 in situ measurements. We have found
that modeled charge-state ratios of C6+/C5+ and
O7+/O6+, among others, were too low compared with
Ulysses measurements. However, a heuristic reduction of the plasma
flow speed has been able to bring the modeled results in line with
observations, though other ideas have been proposed to address this
discrepancy. We discuss implications of our results and the prospect
of including ion charge-state calculations into our 3D MHD model of
the inner heliosphere.
Title: Identifying Observables That Can Differentiate Between
Impulsive and Footpoint Heating: Time Lags and Intensity Ratios
Authors: Winebarger, Amy R.; Lionello, Roberto; Downs, Cooper; Mikić,
Zoran; Linker, Jon
Bibcode: 2018ApJ...865..111W
Altcode: 2018arXiv180605374W
Observations of solar coronal loops have identified several common loop
characteristics, including that loops appear to cool and have higher
than expected densities. Two potential heating scenarios have been
suggested to explain these observations. One scenario is that the loops
are formed by many strands, each heated independently by a series of
small-scale impulsive heating events, or nanoflares. Another hypothesis
is that the heating is quasi-steady and highly stratified, i.e.,
“footpoint heating” such heating can drive thermal nonequilibrium
in some structures depending on the scale height and magnitude of
the energy deposition, and the geometry of the structure. Studies
of both types of heating have found that they can qualitatively
reproduce the observed loop properties. The goal of this paper is to
identify observables that can be used to differentiate between these
two heating scenarios. To do this, we use a single loop geometry. For
footpoint heating, we vary the heating magnitude and stratification, for
impulsive heating, we vary the heating magnitude. We use one-dimensional
hydrodynamic codes to calculate the resulting temperature and density
evolution. We convolve the temperature and density with the response
functions of four EUV channels of the Atmospheric Imaging Assembly
and one filter channel of Hinode's X-ray Telescope. We consider two
principal diagnostics: the time lag between the appearance of the loop
in two different channels, and the ratio of the peak intensities of
the loop in the two channels. Based on this limited data set, we find
(1) that footpoint heating can predict longer time lags than impulsive
heating in some channel pairs, (2) that footpoint heating can predict
zero or negative time lags in some channel pairs, (3) that the intensity
ratio expected from impulsive heating is confined to a narrow range,
while footpoint heating predicts a wider range of intensity ratios,
and (4) that the range of temperatures expected in impulsive heating
is broader than the range of temperatures expected in footpoint
heating. This preliminary study identifies observables that may be
useful in discriminating between heating models in future work.
Title: Global Non-Potential Magnetic Models of the Solar Corona
During the March 2015 Eclipse
Authors: Yeates, Anthony R.; Amari, Tahar; Contopoulos, Ioannis; Feng,
Xueshang; Mackay, Duncan H.; Mikić, Zoran; Wiegelmann, Thomas; Hutton,
Joseph; Lowder, Christopher A.; Morgan, Huw; Petrie, Gordon; Rachmeler,
Laurel A.; Upton, Lisa A.; Canou, Aurelien; Chopin, Pierre; Downs,
Cooper; Druckmüller, Miloslav; Linker, Jon A.; Seaton, Daniel B.;
Török, Tibor
Bibcode: 2018SSRv..214...99Y
Altcode: 2018arXiv180800785Y
Seven different models are applied to the same problem of simulating
the Sun's coronal magnetic field during the solar eclipse on 2015
March 20. All of the models are non-potential, allowing for free
magnetic energy, but the associated electric currents are developed
in significantly different ways. This is not a direct comparison
of the coronal modelling techniques, in that the different models
also use different photospheric boundary conditions, reflecting
the range of approaches currently used in the community. Despite
the significant differences, the results show broad agreement in the
overall magnetic topology. Among those models with significant volume
currents in much of the corona, there is general agreement that the
ratio of total to potential magnetic energy should be approximately
1.4. However, there are significant differences in the electric current
distributions; while static extrapolations are best able to reproduce
active regions, they are unable to recover sheared magnetic fields in
filament channels using currently available vector magnetogram data. By
contrast, time-evolving simulations can recover the filament channel
fields at the expense of not matching the observed vector magnetic
fields within active regions. We suggest that, at present, the best
approach may be a hybrid model using static extrapolations but with
additional energization informed by simplified evolution models. This
is demonstrated by one of the models.
Title: Predicting the corona for the 21 August 2017 total solar
eclipse
Authors: Mikić; , Zoran; Downs, Cooper; Linker, Jon A.; Caplan, Ronald
M.; Mackay, Duncan H.; Upton, Lisa A.; Riley, Pete; Lionello, Roberto;
Török, Tibor; Titov, Viacheslav S.; Wijaya, Janvier; Druckmüller,
Miloslav; Pasachoff, Jay M.; Carlos, Wendy
Bibcode: 2018NatAs...2..913M
Altcode: 2018NatAs.tmp..120M
The total solar eclipse that occurred on 21 August 2017 across the
United States provided an opportunity to test a magnetohydrodynamic
model of the solar corona driven by measured magnetic fields. We used
a new heating model based on the dissipation of Alfvén waves, and
a new energization mechanism to twist the magnetic field in filament
channels. We predicted what the corona would look like one week before
the eclipse. Here, we describe how this prediction was accomplished,
and show that it compared favourably with observations of the
eclipse in white light and extreme ultraviolet. The model allows us to
understand the relationship of observed features, including streamers,
coronal holes, prominences, polar plumes and thin rays, to the magnetic
field. We show that the discrepancies between the model and observations
arise from limitations in our ability to observe the Sun's magnetic
field. Predictions of this kind provide opportunities to improve the
models, forging the path to improved space weather prediction.
Title: Generalizing the RBSL-method for Flux Ropes with Various
Current Profiles and Nonzero External Axial Field
Authors: Titov, Viacheslav; Linker, Jon; Mikic, Zoran; Downs, Cooper;
Torok, Tibor; Caplan, Ronald; Wijaya, Janvier
Bibcode: 2018cosp...42E3391T
Altcode:
Magnetic flux ropes (FRs) likely play a key role in prominence formation
and solar eruptions.It is therefore important to develop methods for
constructing FR configurations constrained by observational data.With
this aim, we have recently derived a pair of regularized Biot-Savart
laws (RBSLs; Titov et al. 2017) that allow one to efficiently calculate
the magnetic vector potential of an FR with circular cross-sections
and an axis of arbitrary shape.One of the RBSLs represents the axial
component of the vector potential produced by the axial current of the
FR, while the other represents the azimuthal component produced by the
axial flux of the FR.The kernels of the RBSLs are regularized at the
axis in such a way that, when the axis is straight, the RBSLs define a
cylindrical flux rope whose structure is exactly force-free.Therefore,
a curved thin FR defined by the RBSLs with the same kernels is
approximately force-free.Originally, we implemented the RBSLs only
for FRs that have a parabolic profile of the axial current and a
vanishing axial magnetic field at the FR surface.Here we present a
two-parametric generalization of the method that describes FRs with
various axial-current profiles and a nonvanishing external axial field
existing in sheared configurations.To benchmark this generalization, we
applied it first to simple configurations of a toroidal-arc FR embedded
into a potential background field, which are geometrically similar to
the model proposed by Titov & Démoulin (1999).We investigated the
numerical FR equilibria reached in zero-beta MHD relaxations of these
configurations in dependence of the initial axial-current profile
and the strength of the external axial field. We plan to apply the
generalized RBSLs to more realistic and complex configurations. Our
previous successful applications of the RBSLs for FRs with a parabolic
axial-current profile suggest the following. The shape of the FR
axis can be determined in more complicated cases by tracking the
observed polarity inversion line of the eruptions' source region and
estimating its height variation as well as other FR parameters by means
of a potential field extrapolated from the observed magnetogram. This
research was supported by NASA's HSR, LWS, and HGI programs,NSF grants
AGS-1560411 and AGS-1135432,and AFOSR contract FA9550-15-C-0001.
Title: Using MHD Simulations for Space-Weather Forecasting: Where
do we Stand?
Authors: Torok, Tibor; Linker, Jon; Mikic, Zoran; Riley, Pete; Titov,
Viacheslav; Lionello, Roberto; Downs, Cooper; Caplan, Ronald; Wijaya,
Janvier
Bibcode: 2018cosp...42E3415T
Altcode:
Coronal mass ejections (CMEs) are the main driver of space-weather
disturbances in the terrestrial magnetosphere. Predicting the impact
of CMEs before they arrive at Earth is one of the main challenges
of solar and heliospheric physics. A candidate tool for this purpose
are numerical simulations. State-of-the-art MHD simulations are now
capable of modeling CMEs all the way from Sun to Earth, but they
are computationally still too demanding to be used for real-time
modeling. At present, only a simplified model (ENLIL), which does not
include the corona and simulates CMEs as velocity perturbations, is used
for operational space-weather forecast. However, given the continuous
increase of computing power, more sophisticated simulations may become
available for this purpose in the near future, and first attempts
are currently made to prepare for operational use. A specific task at
hand is to evaluate the accuracy of these simulations in reproducing
in-situ measurements at Earth. I this presentation, we will briefly
review state-of-the-art CME simulations and discuss their predictive
capabilities and limitations. As an example, we will present a recent
Sun-to-Earth simulation of the well-known 14 July 2000 "Bastille-Day"
event, which produced a very strong geomagnetic storm.
Title: Effect of Coronal Hole Obscuration on Open Flux Measurements
Authors: Caplan, Ronald Meyer; Downs, Cooper; Linker, Jon A.; Mikic,
Zoran
Bibcode: 2018shin.confE.254C
Altcode:
Coronal holes (CH) are commonly associated with open field regions
on the Sun. Full-sun maps of CHs detected in EUV images can be
overlaid on photospheric magnetic field measurements to estimate
the open magnetic flux in the heliosphere, but these estimates are
typically smaller than averaged in-situ measurements at Earth. This
could be due to many factors, including systematic uncertainties in
measured magnetic field strengths, under-estimated polar field values,
parameter/algorithm choices of the CH detection, and incomplete CH
detection due to obscuration by nearby structures. Here we attempt to
systematically test the effects of CH obscuration using diagnostics
from a realistic thermodynamic MHD model of the global solar corona. By
generating synthetic EUV images from multiple viewing perspectives,
we explore how CH maps and open flux diagnostics change depending on
the available viewing geometries (all perspectives, Sun-Earth line
only, and 2-3 views only). These results give insight into how much
the obscuration of CHs might influence CH-based open flux estimates.
Title: Solar EUV Irradiance: a Coronal Modeling Perspective
Authors: Downs, Cooper; Linker, Jon; Mikic, Zoran; Lionello, Roberto
Bibcode: 2018tess.conf40902D
Altcode:
Understanding, modeling, and eventually predicting the response of
the Earth's atmosphere to changes in the Sun's spectral irradiance
naturally requires a cross disciplinary approach to bring together
knowledge, data, and models from the diverse physical systems
involved. In this talk, we will outline what we see as the current
state-of-the-art in physics-based coronal modeling techniques,
particularly those pertaining to forward modeling and predicting the
EUV and soft X-Ray spectral irradiance of the Sun. With the goal
of opening a dialog about what modeling products may (or may not)
be of use to the larger solar irradiance community, we will draw
from a range examples—from our latest research modeling efforts,
to production runs that are routinely available. We will outline the
challenges in modeling the thermal-magnetic structure of solar corona
and its variation, which involves capturing the essential interplay
between coronal heating, plasma dynamics, and the inherently complex,
structured magnetic field of the corona. A particular focus will be
placed on how physics-based coronal modeling and spectral synthesis may
potentially supplement and/or improve atmospheric model drivers based
on irradiance observations and proxies, ideally providing context for
future integrated irradiance and atmospheric modeling efforts.
Title: Partially Open Fields and Solar Eruptions
Authors: Linker, Jon; Mikic, Zoran; Downs, Cooper; Caplan, Ronald M.;
Riley, Pete; Torok, Tibor; Titov, Viacheslav S.; Lionello, Roberto;
Amari, Tahar
Bibcode: 2018tess.conf10905L
Altcode:
Partially Open Fields and Solar Eruptions* Major solar eruptions
such as X-class flares and coronal mass ejections (CMEs) are the
progenitors of solar energetic particles and geomagnetic storms, and are
thus key drivers of space weather at Earth. The solar magnetic field
is the ultimate source of these massive events, the energy of which
is believed to be stored as free magnetic energy (energy above the
potential field state) prior to eruption. The amount of free magnetic
energy available in a given region is therefore a crucial indicator
of its propensity for eruption. However, solar active regions,
from which the largest events originate, can store widely varying
amounts of energy. Therefore, estimates of the free energy alone are
likely to be insufficient for knowing when a region will erupt; we
must also estimate the bounds on how much energy can be stored in a
given region. The Aly-Sturrock theorem (Aly, ApJ 1991; Sturrock,
ApJ 1991) shows that the energy of a fully force-free field cannot
exceed the energy of the so-called open field. If the theorem holds,
this places an upper limit on the amount of free energy that can
be stored. In this paper, we describe how a closely related field,
the partially open field (Wolfson & Low ApJ 1992; Hu, ApJ 2004;
Aly & Amari, GAFD 2007), may place a much tighter bound on energy
storage and yield insights as to when major eruptions from an active
region are imminent (Amari et al., Nature, 2014). We demonstrate
the idea for AR9077, the source of the July 14, 2000 "Bastille Day"
flare/CME. *Research supported by NASA and AFOSR
Title: Sun-To-Earth MHD Simulation of the 14 JULY 2000 "Bastille
Day" Eruption
Authors: Torok, Tibor; Downs, Cooper; Linker, Jon A.; Lionello,
Roberto; Titov, Viacheslav S.; Mikic, Zoran; Riley, Pete; Caplan,
Ron M.; Wijaya, Janvier
Bibcode: 2018EGUGA..20.5564T
Altcode:
Solar eruptions are the main driver of space-weather disturbances at
the Earth. Extreme events are of particular interest, not only because
of the scientific challenges they pose, but also because of their
possible societal consequences. Here we present a magnetohydrodynamic
(MHD) simulation of the 14 July 2000 ``Bastille Day" eruption,
which produced a very strong geomagnetic storm. After constructing a
``thermodynamic" MHD model of the corona and solar wind, we insert a
magnetically stable flux rope along the polarity inversion line of
the eruption's source region and initiate the eruption by boundary
flows. More than 1033 ergs of magnetic energy are released
in the eruption within a few minutes, driving a flare, an EUV wave, and
a coronal mass ejection (CME) that travels in the outer corona at ≈
1500 km s-1, close to the observed speed. We then propagate
the CME to Earth, using a heliospheric MHD code. Our simulation thus
provides the opportunity to test how well in situ observations of
extreme events are matched if the eruption is initiated from a stable
magnetic-equilibrium state. We find that the flux-rope center is very
similar in character to the observed magnetic cloud, but arrives
≈ 8.5 hours later and ≈ 15° too far to the North, with field
strengths that are too weak by a factor of ≈ 1.6. The front of the
flux rope is highly distorted, exhibiting localized magnetic-field
concentrations as it passes 1 AU. We discuss these properties with
regard to the development of space-weather predictions based on MHD
simulations of solar eruptions.
Title: Sun-to-Earth MHD Simulation of the 2000 July 14 “Bastille
Day” Eruption
Authors: Török, Tibor; Downs, Cooper; Linker, Jon A.; Lionello, R.;
Titov, Viacheslav S.; Mikić, Zoran; Riley, Pete; Caplan, Ronald M.;
Wijaya, Janvier
Bibcode: 2018ApJ...856...75T
Altcode: 2018arXiv180105903T
Solar eruptions are the main driver of space-weather disturbances at
Earth. Extreme events are of particular interest, not only because
of the scientific challenges they pose, but also because of their
possible societal consequences. Here we present a magnetohydrodynamic
(MHD) simulation of the 2000 July 14 “Bastille Day” eruption,
which produced a very strong geomagnetic storm. After constructing
a “thermodynamic” MHD model of the corona and solar wind, we
insert a magnetically stable flux rope along the polarity inversion
line of the eruption’s source region and initiate the eruption
by boundary flows. More than 1033 erg of magnetic energy
is released in the eruption within a few minutes, driving a flare,
an extreme-ultraviolet wave, and a coronal mass ejection (CME) that
travels in the outer corona at ≈1500 km s-1, close to the
observed speed. We then propagate the CME to Earth, using a heliospheric
MHD code. Our simulation thus provides the opportunity to test how well
in situ observations of extreme events are matched if the eruption is
initiated from a stable magnetic equilibrium state. We find that the
flux-rope center is very similar in character to the observed magnetic
cloud, but arrives ≈8.5 hr later and ≈15° too far to the north,
with field strengths that are too weak by a factor of ≈1.6. The front
of the flux rope is highly distorted, exhibiting localized magnetic
field concentrations as it passes 1 au. We discuss these properties
with regard to the development of space-weather predictions based on
MHD simulations of solar eruptions.
Title: On the Occurrence of Thermal Nonequilibrium in Coronal Loops
Authors: Froment, C.; Auchère, F.; Mikić, Z.; Aulanier, G.;
Bocchialini, K.; Buchlin, E.; Solomon, J.; Soubrié, E.
Bibcode: 2018ApJ...855...52F
Altcode: 2018arXiv180204010F
Long-period EUV pulsations, recently discovered to be common in active
regions, are understood to be the coronal manifestation of thermal
nonequilibrium (TNE). The active regions previously studied with
EIT/Solar and Heliospheric Observatory and AIA/SDO indicated that
long-period intensity pulsations are localized in only one or two
loop bundles. The basic idea of this study is to understand why. For
this purpose, we tested the response of different loop systems, using
different magnetic configurations, to different stratifications and
strengths of the heating. We present an extensive parameter-space study
using 1D hydrodynamic simulations (1020 in total) and conclude that the
occurrence of TNE requires specific combinations of parameters. Our
study shows that the TNE cycles are confined to specific ranges in
parameter space. This naturally explains why only some loops undergo
constant periodic pulsations over several days: since the loop geometry
and the heating properties generally vary from one loop to another in
an active region, only the ones in which these parameters are compatible
exhibit TNE cycles. Furthermore, these parameters (heating and geometry)
are likely to vary significantly over the duration of a cycle, which
potentially limits the possibilities of periodic behavior. This study
also confirms that long-period intensity pulsations and coronal rain are
two aspects of the same phenomenon: both phenomena can occur for similar
heating conditions and can appear simultaneously in the simulations.
Title: Regularized Biot-Savart Laws for Modeling Magnetic Flux Ropes
Authors: Titov, Viacheslav S.; Downs, Cooper; Mikić, Zoran; Török,
Tibor; Linker, Jon A.; Caplan, Ronald M.
Bibcode: 2018ApJ...852L..21T
Altcode: 2017arXiv171206708T
Many existing models assume that magnetic flux ropes play a key role
in solar flares and coronal mass ejections (CMEs). It is therefore
important to develop efficient methods for constructing flux-rope
configurations constrained by observed magnetic data and the morphology
of the pre-eruptive source region. For this purpose, we have derived
and implemented a compact analytical form that represents the magnetic
field of a thin flux rope with an axis of arbitrary shape and circular
cross-sections. This form implies that the flux rope carries axial
current I and axial flux F, so that the respective magnetic field is the
curl of the sum of axial and azimuthal vector potentials proportional
to I and F, respectively. We expressed the vector potentials in terms
of modified Biot-Savart laws, whose kernels are regularized at the
axis in such a way that, when the axis is straight, these laws define a
cylindrical force-free flux rope with a parabolic profile for the axial
current density. For the cases we have studied so far, we determined
the shape of the rope axis by following the polarity inversion line of
the eruptions’ source region, using observed magnetograms. The height
variation along the axis and other flux-rope parameters are estimated
by means of potential-field extrapolations. Using this heuristic
approach, we were able to construct pre-eruption configurations for
the 2009 February 13 and 2011 October 1 CME events. These applications
demonstrate the flexibility and efficiency of our new method for
energizing pre-eruptive configurations in simulations of CMEs.
Title: Can Polar Fields Explain Missing Open Flux?
Authors: Linker, J.; Downs, C.; Caplan, R. M.; Riley, P.; Mikic, Z.;
Lionello, R.
Bibcode: 2017AGUFMSH54A..02L
Altcode:
The "open" magnetic field is the portion of the Sun's magnetic field
that extends out into the heliosphere and becomes the interplanetary
magnetic field (IMF). Both the IMF and the Sun's magnetic field in
the photosphere have been measured for many years. In the standard
paradigm of coronal structure, the open magnetic field originates
primarily in coronal holes. The regions that are magnetically closed
trap the coronal plasma and give rise to the streamer belt. This basic
picture is qualitatively reproduced by models of coronal structure using
photospheric magnetic fields as input. If this paradigm is correct,
there are two primary observational constraints on the models: (1)
The open field regions in the model should approximately correspond
to coronal holes observed in emission, and (2) the magnitude of the
open magnetic flux in the model should match that inferred from in
situ spacecraft measurements. Linker et al. (2017, ApJ, submitted)
investigated the July 2010 time period for a range of observatory
maps and both PFSS and MHD models. We found that all of the model/map
combinations underestimated the interplanetary magnetic flux, unless the
modeled open field regions were larger than observed coronal holes. An
estimate of the open magnetic flux made entirely from solar observations
(combining detected coronal hole boundaries with observatory synoptic
magnetic maps) also underestimated the interplanetary magnetic flux. The
magnetic field near the Sun's poles is poorly observed and may not
be well represented in observatory maps. In this paper, we explore
whether an underestimate of the polar magnetic flux during this time
period could account for the overall underestimate of open magnetic
flux. Research supported by NASA, AFOSR, and NSF.
Title: The Solar Orbiter Heliospheric Imager (SoloHI) for the Solar
Orbiter Mission
Authors: Howard, R.; Colaninno, R. C.; Plunkett, S. P.; Thernisien,
A. F.; Wang, D.; Rich, N.; Korendyke, C.; Socker, D. G.; Linton, M.;
McMullin, D. R.; Vourlidas, A.; Liewer, P. C.; De Jong, E.; Velli,
M.; Mikic, Z.; Bothmer, V.; Philippe, L.; Carter, M. T.
Bibcode: 2017AGUFMSH23D2681H
Altcode:
The SoloHI instrument has completed its development effort and has been
integrated onto the Solar Orbiter (SolO) spacecraft. The SolO mission,
scheduled for launch in February 2019, will undergo gravity assist
maneuvers around Venus to change both the perihelion distance as well
as the plane of the orbit to ultimately achieve a minimum perihelion
of 0.28 AU and an orbital inclination of about 35° relative to the
ecliptic plane. The remote sensing instruments will operate for three
10-day periods out of the nominal 6-month orbit. SoloHI will observe
sunlight scattered by free electrons in the corona/solar wind from 5°
to 45° elongation in visible wavelengths and will provide a coupling
between remote sensing and in situ observations. It is very similar
to the HI-1 instrument on STEREO/SECCHI except that the FOV is twice
the size at 40o. We present our efforts to prepare for the mission
including our observing plans, quick-look plans and some results of
the calibration activities. We gratefully acknowledge the support of
the NASA Solar Orbiter Collaboration project.
Title: 3D MHD Modeling of Prominence Formation by Plasma Evaporation
and Condensation
Authors: Torok, T.; Lionello, R.; Mikic, Z.; Downs, C.; Titov, V. S.
Bibcode: 2017AGUFMSH41C..07T
Altcode:
The formation of prominence material in the solar corona still belongs
to the open questions of solar physics. There exists a consensus
that prominence plasma has to be of chromospheric origin, but the
mechanisms by which it accumulates in the corona are still not well
understood. The presently most accepted scenario invokes the evaporation
of chromospheric plasma via foot point heating and its subsequent
condensation in the corona via thermal instabilities. This scenario
has been successfully modeled in 1D hydrodynamic simulations along
single field lines of a static magnetic field, but a more appropriate,
fully 3D treatment of the thermodynamics in time-dependent magnetic
fields was started just very recently by Xia et al. Our group at
PSI has recently begun to engage in this challenging task as well,
using our time-dependent, fully 3D thermodynamic MHD code MAS. For
our investigation we consider two different coronal flux-rope
configurations, using the analytical model by Titov and Démoulin and
a model in which an elongated flux rope is constructed by photospheric
flows. We investigate the plasma behavior for both configurations,
using heating models of different complexity, and accompany our analysis
by 1D loop simulations performed along selected field lines. In this
presentation, we outline our modeling approach and discuss the results
obtained so far.
Title: The Wide-Field Imager for the Parker Solar Probe Mission
(WISPR)
Authors: Plunkett, S. P.; Howard, R.; Chua, D. H.; Crump, N. A.;
Dennison, H.; Korendyke, C.; Linton, M.; Rich, N.; Socker, D. G.;
Thernisien, A. F.; Wang, D.; Vourlidas, A.; Baugh, R.; Van Duyne,
J. P.; Liewer, P. C.; De Jong, E.; Boies, M. T.; Mikic, Z.; Bothmer,
V.; Rochus, P.; Halain, J. P.
Bibcode: 2017AGUFMSH23D2693P
Altcode:
The Parker Solar Probe (PSP) mission will be humanity's first visit
to the atmosphere of our nearest star, the Sun, when it is launched
in July 2018. PSP will complete 24 orbits between the Sun and Venus
with diminishing perihelia reaching as close as 7 million km (9.86
solar radii) from Sun center. In addition to a suite of in-situ probes
for the magnetic field, plasma, and energetic particles, the payload
includes the Wide Field Imager for Solar Probe (WISPR) that will
record unprecedented visible light images of the solar corona and the
inner heliosphere. WISPR is the smallest heliospheric imager to date,
and comprises two nested wide-field telescopes with large-format (2K
x 2K) APS CMOS detectors to optimize the performance over a combined
95º radial by 58º transverse field of view and to minimize the risk
of dust damage, which may be considerable close to the Sun. WISPR will
discover - in this never-before explored region of the heliosphere - the
fundamental nature of coronal structures and the source regions of the
solar wind as the PSP flies through them, and will determine whether a
dust-free zone exists near the Sun. WISPR has completed its development
effort and has been integrated onto the PSP spacecraft. In this paper,
we will present our efforts to prepare for the mission including our
observing plans and some results of the calibration activities.
Title: Ion Charge States in the July 14, 2000 CME: MHD Simulations
Authors: Lionello, R.; Riley, P.; Torok, T.; Linker, J.; Mikic, Z.;
Raymond, J. C.; Shen, C.
Bibcode: 2017AGUFMSH11B2438L
Altcode:
In situ measurements of ion fractional charge states at 1 AU and
elsewhere can provide important information about electron temperatures
back in the corona, since, once "frozen in," the charge states remain
essentially unaltered as they travel through the solar wind. For
example, high-ionization states suggest that the plasma originated
from hotter regions on the solar corona. However, connecting these
in situ measurements with remote spectroscopic observations has
proven difficult. Using a global MHD model of the solar corona and
heliosphere, which includes the self-consistent calculation of minor
ion charge states, we compute the fractional charge state profiles of
several ions associated with the CME that occurred on July 14, 2000
and the ambient solar wind. Our approach is based on non-equilibrium
ionization calculations, which are more accurate than the standard
ionization equilibrium way of computing charge states. We follow the
evolution of these profiles, together with the magnetofluid parameters
as the plasma propagates from the low corona to 1 AU. We discuss the
results of the CME simulations, compare them with in situ measurements,
and relate them to theories for the origin of CMEs.
Title: Thermal energy creation and transport and X-ray/EUV emission
in a thermodynamic MHD CME simulation
Authors: Reeves, K.; Mikic, Z.; Torok, T.; Linker, J.; Murphy, N. A.
Bibcode: 2017AGUFMSH11C..07R
Altcode:
We model a CME using the PSI 3D numerical MHD code that includes
coronal heating, thermal conduction and radiative cooling in the
energy equation. The magnetic flux distribution at 1 Rs is produced by
a localized subsurface dipole superimposed on a global dipole field,
mimicking the presence of an active region within the global corona. We
introduce transverse electric fields near the neutral line in the
active region to form a flux rope, then a converging flow is imposed
that causes the eruption. We follow the formation and evolution of
the current sheet and find that instabilities set in soon after the
reconnection commences. We simulate XRT and AIA EUV emission and find
that the instabilities manifest as bright features emanating from the
reconnection region. We examine the quantities responsible for plasma
heating and cooling during the eruption, including thermal conduction,
radiation, adiabatic compression and expansion, coronal heating and
ohmic heating due to dissipation of currents. We find that the adiabatic
compression plays an important role in heating the plasma around the
current sheet, especially in the later stages of the eruption when the
instabilities are present. Thermal conduction also plays an important
role in the transport of thermal energy away from the current sheet
region throughout the reconnection process.
Title: Regularized Biot-Savart Laws for Modeling Magnetic
Configurations with Flux Ropes
Authors: Titov, V. S.; Downs, C.; Mikic, Z.; Torok, T.; Linker, J.
Bibcode: 2017AGUFMSH12A..06T
Altcode:
Many existing models assume that magnetic flux ropes play a key role
in solar flares and coronal mass ejections (CMEs). It is therefore
important to develop efficient methods for constructing flux-rope
configurations constrained by observed magnetic data and the initial
morphology of CMEs. For this purpose, we have derived and implemented
a compact analytical form that represents the magnetic field of
a thin flux rope with an axis of arbitrary shape and a circular
cross-section. This form implies that the flux rope carries axial
current I and axial flux F, so that the respective magnetic field is the
curl of the sum of toroidal and poloidal vector potentials proportional
to I and F, respectively. We expressed the vector potentials in terms
of modified Biot-Savart laws whose kernels are regularized at the axis
in such a way that these laws define a cylindrical force-free flux
rope with a parabolic profile of the axial current density, when the
axis is straight. For the cases we have studied so far, we determined
the shape of the rope axis by following the polarity inversion line of
the eruptions' source region, using observed magnetograms. The height
variation along the axis and other flux-rope parameters are estimated
by means of potential field extrapolations. Using this heuristic
approach, we were able to construct pre-eruption configurations for
the 2009 February13 and 2011 October 1 CME events. These applications
demonstrate the flexibility and efficiency of our new method for
energizing pre-eruptive configurations in MHD simulations of CMEs. We
discuss possible ways of optimizing the axis paths and other extensions
of the method in order to make it more useful and robust. Research
supported by NSF, NASA's HSR and LWS Programs, and AFOSR.
Title: Modeling the 21 August 2017 Total Solar Eclipse: Prediction
Results and New Techniques
Authors: Downs, C.; Mikic, Z.; Caplan, R. M.; Linker, J.; Lionello,
R.; Torok, T.; Titov, V. S.; Riley, P.; MacKay, D.; Upton, L.
Bibcode: 2017AGUFMSH13B2475D
Altcode:
As has been our tradition for past solar eclipses, we conducted a
high resolution magnetohydrodynamic (MHD) simulation of the corona
to predict the appearance of the 21 August 2017 solar eclipse. In
this presentation, we discuss our model setup and our forward
modeled predictions for the corona's appearance, including images
of polarized brightness and EUV/soft X-Ray emission. We show how
the combination of forward modeled observables and knowledge of the
underlying magnetic field from the model can be used to interpret
the structures seen during the eclipse. We also discuss two new
features added to this year's prediction. First, in an attempt to
improve the morphological shape of streamers in the low corona,
we energize the large-scale magnetic field by emerging shear and
canceling flux within filament channels. The handedness of the shear
is deduced from a magnetofrictional model, which is driven by the
evolving photospheric field produced by the Advective Flux Transport
model. Second, we apply our new wave-turbulence-driven (WTD) model for
coronal heating. This model has substantially fewer free parameters
than previous empirical heating models, but is inherently sensitive to
the 3D geometry and connectivity of the magnetic field--a key property
for modeling the thermal-magnetic structure of the corona. We examine
the effect of these considerations on forward modeled observables,
and present them in the context of our final 2017 eclipse prediction
(www.predsci.com/corona/aug2017eclipse). Research supported by NASA's
Heliophysics Supporting Research and Living With a Star Programs.
Title: Time-dependent Ionization in a Steady Flow in an MHD Model
of the Solar Corona and Wind
Authors: Shen, Chengcai; Raymond, John C.; Mikić, Zoran; Linker,
Jon A.; Reeves, Katharine K.; Murphy, Nicholas A.
Bibcode: 2017ApJ...850...26S
Altcode:
Time-dependent ionization is important for diagnostics of coronal
streamers and pseudostreamers. We describe time-dependent ionization
calculations for a three-dimensional magnetohydrodynamic (MHD) model of
the solar corona and inner heliosphere. We analyze how non-equilibrium
ionization (NEI) influences emission from a pseudostreamer during the
Whole Sun Month interval (Carrington rotation CR1913, 1996 August 22
to September 18). We use a time-dependent code to calculate NEI states,
based on the plasma temperature, density, velocity, and magnetic field
in the MHD model, to obtain the synthetic emissivities and predict
the intensities of the Lyα, O VI, Mg x, and Si xii emission lines
observed by the SOHO/Ultraviolet Coronagraph Spectrometer (UVCS). At
low coronal heights, the predicted intensity profiles of both Lyα
and O VI lines match UVCS observations well, but the Mg x and Si xii
emission are predicted to be too bright. At larger heights, the O VI
and Mg x lines are predicted to be brighter for NEI than equilibrium
ionization around this pseudostreamer, and Si xii is predicted to be
fainter for NEI cases. The differences of predicted UVCS intensities
between NEI and equilibrium ionization are around a factor of 2, but
neither matches the observed intensity distributions along the full
length of the UVCS slit. Variations in elemental abundances in closed
field regions due to the gravitational settling and the FIP effect may
significantly contribute to the predicted uncertainty. The assumption
of Maxwellian electron distributions and errors in the magnetic field
on the solar surface may also have notable effects on the mismatch
between observations and model predictions.
Title: Particle Radiation Sources, Propagation and Interactions
in Deep Space, at Earth, the Moon, Mars, and Beyond: Examples of
Radiation Interactions and Effects
Authors: Schwadron, Nathan A.; Cooper, John F.; Desai, Mihir; Downs,
Cooper; Gorby, Matt; Jordan, Andrew P.; Joyce, Colin J.; Kozarev,
Kamen; Linker, Jon A.; Mikíc, Zoran; Riley, Pete; Spence, Harlan E.;
Török, Tibor; Townsend, Lawrence W.; Wilson, Jody K.; Zeitlin, Cary
Bibcode: 2017SSRv..212.1069S
Altcode: 2017SSRv..tmp...63S
Particle radiation has significant effects for astronauts, satellites
and planetary bodies throughout the Solar System. Acute space radiation
hazards pose risks to human and robotic exploration. This radiation also
naturally weathers the exposed surface regolith of the Moon, the two
moons of Mars, and other airless bodies, and contributes to chemical
evolution of planetary atmospheres at Earth, Mars, Venus, Titan, and
Pluto. We provide a select review of recent areas of research covering
the origin of SEPs from coronal mass ejections low in the corona,
propagation of events through the solar system during the anomalously
weak solar cycle 24 and important examples of radiation interactions
for Earth, other planets and airless bodies such as the Moon.
Title: The Open Flux Problem
Authors: Linker, J. A.; Caplan, R. M.; Downs, C.; Riley, P.; Mikic,
Z.; Lionello, R.; Henney, C. J.; Arge, C. N.; Liu, Y.; Derosa, M. L.;
Yeates, A.; Owens, M. J.
Bibcode: 2017ApJ...848...70L
Altcode: 2017arXiv170802342L
The heliospheric magnetic field is of pivotal importance in solar
and space physics. The field is rooted in the Sun’s photosphere,
where it has been observed for many years. Global maps of the solar
magnetic field based on full-disk magnetograms are commonly used as
boundary conditions for coronal and solar wind models. Two primary
observational constraints on the models are (1) the open field regions
in the model should approximately correspond to coronal holes (CHs)
observed in emission and (2) the magnitude of the open magnetic
flux in the model should match that inferred from in situ spacecraft
measurements. In this study, we calculate both magnetohydrodynamic and
potential field source surface solutions using 14 different magnetic
maps produced from five different types of observatory magnetograms,
for the time period surrounding 2010 July. We have found that for
all of the model/map combinations, models that have CH areas close
to observations underestimate the interplanetary magnetic flux, or,
conversely, for models to match the interplanetary flux, the modeled
open field regions are larger than CHs observed in EUV emission. In
an alternative approach, we estimate the open magnetic flux entirely
from solar observations by combining automatically detected CHs for
Carrington rotation 2098 with observatory synoptic magnetic maps. This
approach also underestimates the interplanetary magnetic flux. Our
results imply that either typical observatory maps underestimate the
Sun’s magnetic flux, or a significant portion of the open magnetic
flux is not rooted in regions that are obviously dark in EUV and
X-ray emission.
Title: Where is the Open Flux?
Authors: Linker, Jon A.; Downs, Cooper; Caplan, Ronald M.; Lionello,
Roberto; Mikic, Zoran; Riley, Pete; Henney, Carl John; Arge, Charles;
Owens, Matthew
Bibcode: 2017SPD....4830103L
Altcode:
The Sun’s magnetic field has been observed in the photosphere from
ground- and space-based observatories for many years. Global maps of
the solar magnetic field based on full disk magnetograms (either built
up over a solar rotation, or evolved using flux transport models)
are commonly used as boundary conditions for coronal and solar wind
models. Maps from different observatories typically agree qualitatively
but often disagree quantitatively. Estimation of the coronal/solar
wind physics can range from potential field source surface (PFSS)
models with empirical prescriptions to magnetohydrodynamic (MHD)
models with realistic energy transport and sub-grid scale descriptions
of heating and acceleration. Two primary observational constraints
on the models are (1) The open field regions in the model should
approximately correspond to coronal holes observed in emission, and (2)
the magnitude of the open magnetic flux in the model should match that
inferred from in situ spacecraft measurements. We have investigated
the July 2010 time period, using PFSS and MHD models computed using
several available magnetic maps, coronal hole boundaries detected from
STEREO and SDO EUV observations, and estimates of the interplanetary
magnetic flux from in situ ACE measurements. We show that for all
the model/map combinations, models that agree for (1) underestimate
the interplanetary magnetic flux, or, conversely, for models to match
(2), the modeled open field regions are larger than observed coronal
holes. Alternatively, we estimate the open magnetic flux entirely from
solar observations by combining detected coronal hole boundaries with
observatory synoptic magnetic maps, and show that this method also
underestimates the interplanetary magnetic flux. We discuss possible
resolutions.Research supported by NASA, AFOSR, and NSF.
Title: New Techniques Used in Modeling the 2017 Total Solar Eclipse:
Energizing and Heating the Large-Scale Corona
Authors: Downs, Cooper; Mikic, Zoran; Linker, Jon A.; Caplan, Ronald
M.; Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Riley, Pete;
Mackay, Duncan; Upton, Lisa
Bibcode: 2017SPD....4820802D
Altcode:
Over the past two decades, our group has used a magnetohydrodynamic
(MHD) model of the corona to predict the appearance of total solar
eclipses. In this presentation we detail recent innovations and
new techniques applied to our prediction model for the August 21,
2017 total solar eclipse. First, we have developed a method for
capturing the large-scale energized fields typical of the corona,
namely the sheared/twisted fields built up through long-term processes
of differential rotation and flux-emergence/cancellation. Using
inferences of the location and chirality of filament channels (deduced
from a magnetofrictional model driven by the evolving photospheric
field produced by the Advective Flux Transport model), we tailor a
customized boundary electric field profile that will emerge shear along
the desired portions of polarity inversion lines (PILs) and cancel flux
to create long twisted flux systems low in the corona. This method
has the potential to improve the morphological shape of streamers in
the low solar corona. Second, we apply, for the first time in our
eclipse prediction simulations, a new wave-turbulence-dissipation
(WTD) based model for coronal heating. This model has substantially
fewer free parameters than previous empirical heating models, but is
inherently sensitive to the 3D geometry and connectivity of the coronal
field---a key property for modeling/predicting the thermal-magnetic
structure of the solar corona. Overall, we will examine the effect
of these considerations on white-light and EUV observables from the
simulations, and present them in the context of our final 2017 eclipse
prediction model.Research supported by NASA's Heliophysics Supporting
Research and Living With a Star Programs.
Title: Using Coronal Hole Maps to Constrain MHD Models
Authors: Caplan, Ronald M.; Downs, Cooper; Linker, Jon A.; Mikic, Zoran
Bibcode: 2017SPD....4810607C
Altcode:
In this presentation, we explore the use of coronal hole maps (CHMs)
as a constraint for thermodynamic MHD models of the solar corona. Using
our EUV2CHM software suite (predsci.com/chd), we construct CHMs from
SDO/AIA 193Å and STEREO-A/EUVI 195Å images for multiple Carrington
rotations leading up to the August 21st, 2017 total solar eclipse. We
then contruct synoptic CHMs from synthetic EUV images generated
from global thermodynamic MHD simulations of the corona for each
rotation. Comparisons of apparent coronal hole boundaries and estimates
of the net open flux are used to benchmark and constrain our MHD model
leading up to the eclipse. Specifically, the comparisons are used
to find optimal parameterizations of our wave turbulence dissipation
(WTD) coronal heating model.
Title: Regularized Biot-Savart Laws for Modeling Magnetic Flux Ropes
Authors: Titov, Viacheslav; Downs, Cooper; Mikic, Zoran; Torok, Tibor;
Linker, Jon A.
Bibcode: 2017SPD....4840606T
Altcode:
Many existing models assume that magnetic flux ropes play a key role
in solar flares and coronal mass ejections (CMEs). It is therefore
important to develop efficient methods for constructing flux-rope
configurations constrained by observed magnetic data and the initial
morphology of CMEs. As our new step in this direction, we have derived
and implemented a compact analytical form that represents the magnetic
field of a thin flux rope with an axis of arbitrary shape and a circular
cross-section. This form implies that the flux rope carries axial
current I and axial flux F, so that the respective magnetic field is a
curl of the sum of toroidal and poloidal vector potentials proportional
to I and F, respectively. The vector potentials are expressed in terms
of Biot-Savart laws whose kernels are regularized at the rope axis. We
regularized them in such a way that for a straight-line axis the form
provides a cylindrical force-free flux rope with a parabolic profile of
the axial current density. So far, we set the shape of the rope axis
by tracking the polarity inversion lines of observed magnetograms and
estimating its height and other parameters of the rope from a calculated
potential field above these lines. In spite of this heuristic approach,
we were able to successfully construct pre-eruption configurations for
the 2009 February13 and 2011 October 1 CME events. These applications
demonstrate that our regularized Biot-Savart laws are indeed a very
flexible and efficient method for energizing initial configurations
in MHD simulations of CMEs. We discuss possible ways of optimizing
the axis paths and other extensions of the method in order to make it
more useful and robust.Research supported by NSF, NASA's HSR and LWS
Programs, and AFOSR.
Title: Q-Maps of the Solar Corona for Two Solar Cycles - 1996-2017
Authors: Hoeksema, Jon Todd; Liu, Yang; Sun, Xudong; Titov, Viacheslav;
Mikic, Zoran
Bibcode: 2017SPD....4830105H
Altcode:
Maps of magnetic field structures called Q-maps characterize the
changing geometry of the solar corona. The geometrical Q parameter
describes the ‘squashing factor’ of elemental flux tubes. Q-maps
are computed from models based on observations of the photospheric
magnetic field and provide intuitive visualization of large-scale
magnetic topological structures where reconnection preferably occurs. We
have applied the method developed by Titov et al. (e.g. ApJ, 2008)
to standard synoptic maps from SDO/HMI and SOHO/MDI and are computing
daily-update synoptic frames to characterize coronal field evolution
for more than twenty years, from 1996 - 2017. We are making available
the vector magnetic field and value of signed log Q at ten or more
heights from 1.001 to 2.5 Rs computed using the PFSS (Potential Field -
Source Surface) model and for some rotations at greater height using
other coronal field models. Maps showing foot points of computed open
field regions are also provided.
Title: Prediction of the Solar Corona for the 2017 August 21 Total
Solar Eclipse
Authors: Mikic, Zoran; Downs, Cooper; Linker, Jon A.; Caplan, Ronald
M.; Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Riley, Pete;
Mackay, Duncan; Upton, Lisa
Bibcode: 2017SPD....4820801M
Altcode:
It has become our tradition to predict the structure of the corona
prior to eclipses, using a magnetohydrodynamic (MHD) model based on
measurements of photospheric magnetic fields on the Sun. We plan to
continue this tradition for the August 21, 2017 total solar eclipse that
will sweep across the United States. We will predict the structure of
the corona using SDO/HMI photospheric magnetic field data, including
images of polarization brightness, magnetic field line traces, and
images of simulated emission in EUV and X-rays. These images can be
compared directly with observations of the total eclipse, as well as
observations from SDO/AIA, Hinode/XRT, and STEREO/EUVI. This year we
will attempt to energize the magnetic field within filament channels
for a more realistic prediction, by constructing flux ropes at the
locations where filament channels are observed. The handedness of the
flux ropes will be deduced from a magnetofrictional model driven by the
evolving photospheric field produced by the Advective Flux Transport
model.Research supported by NASA's Heliophysics Supporting Research
and Living With a Star Programs.
Title: 2010 August 1-2 Sympathetic Eruptions. II. Magnetic Topology
of the MHD Background Field
Authors: Titov, Viacheslav S.; Mikić, Zoran; Török, Tibor; Linker,
Jon A.; Panasenco, Olga
Bibcode: 2017ApJ...845..141T
Altcode: 2017arXiv170707773T
Using a potential field source-surface (PFSS) model, we recently
analyzed the global topology of the background coronal magnetic field
for a sequence of coronal mass ejections (CMEs) that occurred on
2010 August 1-2. Here we repeat this analysis for the background field
reproduced by a magnetohydrodynamic (MHD) model that incorporates plasma
thermodynamics. As for the PFSS model, we find that all three CME source
regions contain a coronal hole (CH) that is separated from neighboring
CHs by topologically very similar pseudo-streamer structures. However,
the two models yield very different results for the size, shape,
and flux of the CHs. We find that the helmet-streamer cusp line,
which corresponds to a source-surface null line in the PFSS model,
is structurally unstable and does not form in the MHD model. Our
analysis indicates that, generally, in MHD configurations, this line
instead consists of a multiple-null separator passing along the edge
of disconnected-flux regions. Some of these regions are transient
and may be the origin of the so-called streamer blobs. We show that
the core topological structure of such blobs is a three-dimensional
“plasmoid” consisting of two conjoined flux ropes of opposite
handedness, which connect at a spiral null point of the magnetic
field. Our analysis reveals that such plasmoids also appear in
pseudo-streamers on much smaller scales. These new insights into the
coronal magnetic topology provide some intriguing implications for solar
energetic particle events and for the properties of the slow solar wind.
Title: Ion Charge States in the Solar Wind and Coronal Mass Ejections:
MHD Simulations
Authors: Lionello, Roberto; Riley, Pete; Mikic, Zoran; Linker, Jon;
Reeves, Katharine; Shen, Chengcai
Bibcode: 2017shin.confE..18L
Altcode:
In situ measurements of ion fractional charge states at 1 AU and
elsewhere can provide important information about electron temperatures
back in the corona, since, once "frozen in," the charge states remain
essentially unaltered as they travel through the solar wind. For
example, high-ionization states suggest that the plasma originated
from hotter regions on the solar corona. However, connecting these
in situ measurements with remote spectroscopic observations has
proven difficult. Using a global MHD model of the solar corona and
heliosphere, which includes the self-consistent calculation of minor
ion charge states, we compute the fractional charge state profiles of
several ions in both the steady state solar wind and within coronal
mass ejections. Our approach is based on non-equilibrium ionization
calculations, which are more accurate than the standard ionization
equilibrium way of computing charge states. We follow the evolution
of these profiles, together with the magnetofluid parameters as the
plasma propagates from the low corona to 1 AU. We discuss the results
of both steady-state solutions as well as idealized CME simulations,
compare them with in situ measurements, and relate them to theories
for the origin of both the slow solar wind and CMEs.
Title: Q-Maps: A Synoptic Data Product for Investigating Coronal
Connectivity
Authors: Sun, Xudong; Hoeksema, Todd; Liu, Yang; Mikic, Zoran; Titov,
Viacheslav
Bibcode: 2017shin.confE..73S
Altcode:
No abstract at ADS
Title: Open Magnetic Flux and the Slow Solar Wind
Authors: Linker, Jon A.; Downs, Cooper; Caplan, Ronald M.; Riley,
Pete; Mikic, Zoran; Lionello, Roberto; Henney, Carl; Arge, Charles N.;
Owens, Matt
Bibcode: 2017shin.confE..71L
Altcode:
In the standard paradigm of coronal structure (most applicable to solar
minimum), the open magnetic field originates primarily in coronal holes,
regions of low intensity emission in EUV and X-rays. The regions that
are magnetically closed trap the coronal plasma and give rise to the
streamer belt that is prominent in coronagraph and eclipse images. If
this paradigm is correct, then coronal models using global maps of
the photospheric magnetic field should be able to approximately
match coronal boundaries observed in emission, as well as the
magnitude of the open magnetic flux inferred from in situ spacecraft
measurements. We have investigated the July 2010 time period, using
PFSS and MHD models computed using several available magnetic maps,
coronal hole boundaries detected from STEREO and SDO EUV observations,
and estimates of the interplanetary magnetic flux from in situ ACE
measurements. We have found that for all of the model/map combinations,
models that have coronal hole areas close to observations underestimate
the interplanetary magnetic flux, or, conversely, for models to match
the interplanetary flux, the modeled open field regions are larger than
coronal holes observed in EUV emission. In an alternative approach,
we estimate the open magnetic flux entirely from solar observations by
combining automatically detected coronal holes for Carrington rotation
2098 with observatory synoptic magnetic maps for this time period. We
find that this method also underestimates the interplanetary magnetic
flux. We discuss these results and possible implications for the origin
of the slow solar wind.
Title: Advancing parabolic operators in thermodynamic MHD models:
Explicit super time-stepping versus implicit schemes with Krylov
solvers
Authors: Caplan, R. M.; Mikić, Z.; Linker, J. A.; Lionello, R.
Bibcode: 2017JPhCS.837a2016C
Altcode: 2016arXiv161001265C
We explore the performance and advantages/disadvantages of using
unconditionally stable explicit super time-stepping (STS) algorithms
versus implicit schemes with Krylov solvers for integrating parabolic
operators in thermodynamic MHD models of the solar corona. Specifically,
we compare the second-order Runge-Kutta Legendre (RKL2) STS
method with the implicit backward Euler scheme computed using the
preconditioned conjugate gradient (PCG) solver with both a point-Jacobi
and a non-overlapping domain decomposition ILU0 preconditioner. The
algorithms are used to integrate anisotropic Spitzer thermal conduction
and artificial kinematic viscosity at time-steps much larger than
classic explicit stability criteria allow. A key component of the
comparison is the use of an established MHD model (MAS) to compute
a real-world simulation on a large HPC cluster. Special attention is
placed on the parallel scaling of the algorithms. It is shown that,
for a specific problem and model, the RKL2 method is comparable or
surpasses the implicit method with PCG solvers in performance and
scaling, but suffers from some accuracy limitations. These limitations,
and the applicability of RKL methods are briefly discussed.
Title: Long-period Intensity Pulsations in Coronal Loops Explained
by Thermal Non-equilibrium Cycles
Authors: Froment, C.; Auchère, F.; Aulanier, G.; Mikić, Z.;
Bocchialini, K.; Buchlin, E.; Solomon, J.
Bibcode: 2017ApJ...835..272F
Altcode: 2017arXiv170101309F
In solar coronal loops, thermal non-equilibrium (TNE) is a phenomenon
that can occur when the heating is both highly stratified and
quasi-constant. Unambiguous observational identification of TNE
would thus permit us to strongly constrain heating scenarios. While
TNE is currently the standard interpretation of coronal rain, the
long-term periodic evolution predicted by simulations has never been
observed. However, the detection of long-period intensity pulsations
(periods of several hours) has been recently reported with the Solar
and Heliospheric Observatory/EIT, and this phenomenon appears to be very
common in loops. Moreover, the three intensity-pulsation events that we
recently studied with the Solar Dynamics Observatory/Atmospheric Imaging
Assembly (AIA) show strong evidence for TNE in warm loops. In this
paper, a realistic loop geometry from linear force-free field (LFFF)
extrapolations is used as input to 1D hydrodynamic simulations. Our
simulations show that, for the present loop geometry, the heating has
to be asymmetrical to produce TNE. We analyze in detail one particular
simulation that reproduces the average thermal behavior of one of the
pulsating loop bundle observed with AIA. We compare the properties of
this simulation with those deduced from the observations. The magnetic
topology of the LFFF extrapolations points to the presence of sites
of preferred reconnection at one footpoint, supporting the presence
of asymmetric heating. In addition, we can reproduce the temporal
large-scale intensity properties of the pulsating loops. This simulation
further strengthens the interpretation of the observed pulsations as
signatures of TNE. This consequently provides important information
on the heating localization and timescale for these loops.
Title: Core Dimming Regions as Probes of Magnetic Connectivity and
Reconfiguration.
Authors: Downs, C.; Titov, V. S.; Jiong, Q.; Torok, T.; Linker, J.;
Mikic, Z.
Bibcode: 2016AGUFMSH12B..05D
Altcode:
The early onset and evolution of a Coronal Mass Ejection (CME)
is a process that features essential coupling between the erupting
flux-system and the ambient corona. In this presentation we will
discuss the deep coronal dimming signatures of three contrasting
case-study events, and relate these signatures to the pre-event magnetic
configuration. We model each event by inserting a stable flux-rope
into the erupting region and then relaxing the configuration with
a full-sun zero-beta MHD model. Structural analysis of the magnetic
field, including maps of the squashing factor (Q), field line heights,
and the overall connectivity can be used to paint a detailed picture
of the likely eruption process, including where and why deep dimming
features appear. We argue that such features are likely probes of the
reconnection process between erupting magnetic flux and surrounding
coronal magnetic fields--a process relevant to understanding the
dynamic magnetic connectivity of CMEs and flux-ropes in the heliosphere.
Title: Data-Driven Models of the Solar Corona and Solar Wind
Authors: Linker, J.; Downs, C.; Caplan, R. M.; Lionello, R.; Mikic,
Z.; Riley, P.; Henney, C. J.; Arge, C. N.
Bibcode: 2016AGUFMSM32A..02L
Altcode:
MHD simulations of the solar corona using boundary conditions
derived from global maps of the photospheric magnetic field have been
demonstrated to describe many aspects of coronal structure. However,
these models are typically integrated to steady state, using synoptic
or daily-updated magnetic maps to derive the boundary conditions. The
Sun's magnetic flux is always evolving, and these changes in the flux
affect the structure and dynamics of the corona and heliosphere. Flux
transport models can in principle provide a more accurate specification
of the global field distribution, by estimating the likely state of the
photospheric magnetic field on unobserved portions of the Sun. The Air
Force Data Assimilative Photospheric flux Transport (ADAPT) model (Arge
et al. 2010) is well suited for this purpose. In this presentation,
we describe an approach to evolutionary models of the corona and solar
wind, using time-dependent boundary conditions based on ADAPT maps. We
discuss how such modeling may capture inherent time-dependent features
of the corona, as well as some of the data challenges that arise from
this approach.
Title: Closed-field Coronal Heating Driven by Wave Turbulence
Authors: Downs, Cooper; Lionello, Roberto; Mikić, Zoran; Linker,
Jon A.; Velli, Marco
Bibcode: 2016ApJ...832..180D
Altcode: 2016arXiv161002113D
To simulate the energy balance of coronal plasmas on macroscopic scales,
we often require the specification of the coronal heating mechanism
in some functional form. To go beyond empirical formulations and to
build a more physically motivated heating function, we investigate
the wave-turbulence-driven (WTD) phenomenology for the heating of
closed coronal loops. Our implementation is designed to capture the
large-scale propagation, reflection, and dissipation of wave turbulence
along a loop. The parameter space of this model is explored by solving
the coupled WTD and hydrodynamic evolution in 1D for an idealized
loop. The relevance to a range of solar conditions is also established
by computing solutions for over one hundred loops extracted from a
realistic 3D coronal field. Due to the implicit dependence of the WTD
heating model on loop geometry and plasma properties along the loop and
at the footpoints, we find that this model can significantly reduce
the number of free parameters when compared to traditional empirical
heating models, and still robustly describe a broad range of quiet-Sun
and active region conditions. The importance of the self-reflection
term in producing relatively short heating scale heights and thermal
nonequilibrium cycles is also discussed.
Title: Radiation Environments for Future Human Exploration Throughout
the Solar System.
Authors: Schwadron, N.; Gorby, M.; Linker, J.; Riley, P.; Torok,
T.; Downs, C.; Spence, H. E.; Desai, M. I.; Mikic, Z.; Joyce, C. J.;
Kozarev, K. A.; Townsend, L. W.; Wimmer-Schweingruber, R. F.
Bibcode: 2016AGUFMSA41B2371S
Altcode:
Acute space radiation hazards pose one of the most serious risks to
future human and robotic exploration. The ability to predict when and
where large events will occur is necessary in order to mitigate their
hazards. The largest events are usually associated with complex sunspot
groups (also known as active regions) that harbor strong, stressed
magnetic fields. Highly energetic protons accelerated very low in the
corona by the passage of coronal mass ejection (CME)-driven compressions
or shocks and from flares travel near the speed of light, arriving
at Earth minutes after the eruptive event. Whether these particles
actually reach Earth, the Moon, Mars (or any other point) depends on
their transport in the interplanetary magnetic field and their magnetic
connection to the shock. Recent contemporaneous observations during
the largest events in almost a decade show the unique longitudinal
distributions of this ionizing radiation broadly distributed from
sources near the Sun and yet highly isolated during the passage of CME
shocks. Over the last decade, we have observed space weather events
as the solar wind exhibits extremely low densities and magnetic field
strengths, representing states that have never been observed during the
space age. The highly abnormal solar activity during cycles 23 and 24
has caused the longest solar minimum in over 80 years and continues
into the unusually small solar maximum of cycle 24. As a result of
the remarkably weak solar activity, we have also observed the highest
fluxes of galactic cosmic rays in the space age and relatively small
particle radiation events. We have used observations from LRO/CRaTER to
examine the implications of these highly unusual solar conditions for
human space exploration throughout the inner solar system. While these
conditions are not a show-stopper for long-duration missions (e.g., to
the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains
a significant and worsening factor that limits mission durations. If the
heliospheric magnetic field continues to weaken over time, as is likely,
then allowable mission durations will decrease correspondingly. Thus, we
examine the rapidly changing radiation environment and its implications
for human exploration destinations throughout the inner solar system.
Title: The Impact of Coronal Jets on the Solar Wind and Magnetic
Structures in the Inner Heliosphere.
Authors: Lionello, R.; Torok, T.; Titov, V. S.; Linker, J.; Mikic,
Z.; Leake, J. E.; Linton, M.
Bibcode: 2016AGUFMSH53A..06L
Altcode:
Transient, collimated plasma eruptions, so-called coronal (or X-ray)
jets, are observed low in the corona in EUV and soft X-ray bands. They
are thought to be triggered by reconnection between closed and open
magnetic fields, although their formation mechanisms are not yet
fully understood. However, coronal jets are also observed to extend
to several solar radii, suggesting that they may provide a still
undetermined contribution to the solar wind. We simulate coronal jets
with our "thermodynamic" full MHD model of the solar corona by driving
the emergence of a magnetic flux rope into an open coronal magnetic
field. We study the impact of jets to the solar wind by varying the
field strength of the emerging flux rope, and we follow the propagation
of ejected magnetic structures into the inner heliosphere.
Title: Sun-to-Earth MHD Modeling of Powerful Solar Eruptions
Authors: Torok, T.; Downs, C.; Linker, J.; Lionello, R.; Titov, V. S.;
Riley, P.; Mikic, Z.
Bibcode: 2016AGUFMSH14A..05T
Altcode:
Large solar eruptions that produce strong flares and powerful coronal
mass ejections are the main driver of space weather disturbances close
to the Earth. One of the main goals of numerical simulations of such
events is therefore to reproduce their in-situ signatures at 1 AU.This
requires a sophisticated model of the pre-eruptive configuration, the
initiation and early evolution of the eruption, and the large-scale
magnetic and plasma environment in which the eruption propagates. We
have been conducting magnetohydrodynamic (MHD) simulations that comply
with these requirements. We first produce a steady-state MHD solution
of the background corona that incorporates photospheric magnetic field
measurements, realistic energy transfer in the corona, and the solar
wind. We then use the recently developed, modified flux-rope model by
Titov et al. to insert a stable flux rope into the source region of the
eruption, while preserving the original magnetogram. Several instances
of the model can be combined to account for source regions with a highly
curved and elongated polarity inversion line (PIL). The eruption is
then initiated by imposing plasma flows that slowly converge towards
the PIL. Finally, we propagate the eruption to Earth, by coupling the
coronal simulation to our heliospheric MHD code. In this presentation
we illustrate our method for the famous "Bastille Day" event of July
14, 2000, which produced an X5.7 flare, a fast halo CME, andan intense
geomagnetic storm. We assess the quality of the simulation by comparing
synthetic satellite images with the observations, and we discuss how
well it reproduces the in-situ measurements at 1 AU. We also briefly
present our ongoing modeling effort for the more recent event of July
12, 2012, which was observed in great detail all the way from Sun
to Earth.
Title: A Thin-Flux-Rope Approximation as a Basis for Modeling of Pre-
and Post-Eruptive Magnetic Configurations
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J.
Bibcode: 2016AGUFMSH13C2313T
Altcode:
Many existing models of solar flares and coronal mass ejections (CMEs)
assume a key role of magnetic flux ropes in these phenomena. It is
therefore important to have efficient methods for constructing flux-rope
configurations consistent with the observed photospheric magnetic data
and morphology of CMEs. As our new step in this direction, we propose
an analytical formulation that succinctly represents the magnetic
field of a thin flux rope, which has an axis of arbitrary shape and
a circular cross-section with the diameter slowly varying along the
axis. This representation implies also that the flux rope carries
axial current I and axial flux F, so that the respective magnetic
field is a curl of the sum of toroidal and poloidal vector potentials
proportional to I and F, respectively. Each of the two potentials
is individually expressed in terms of a modified Biot-Savart law
with separate kernels, both regularized at the rope axis. We argue
that the proposed representation is flexible enough to be used in
MHD simulations for initializing pre-eruptive configurations in the
low corona or post-eruptive configurations (interplanetary CMEs) in
the heliosphere. We discuss the potential advantages of our approach,
and the subsequent steps to be performed, to develop a fully operative
and highly competitive method compared to existing methods. Research
supported by NSF, NASA's HSR and LWS Programs, and AFOSR.
Title: Q-Maps: A New Synoptic Data Product for Investigating Dynamic
Coronal Connectivity
Authors: Sun, X.; Hoeksema, J. T.; Liu, Y.; Mikic, Z.; Titov, V. S.
Bibcode: 2016AGUFMSH13C2312S
Altcode:
The "squashing degree" Q parameter characterizes the local divergence
of nearby magnetic field lines. It highlights separatrix and
quasi-separatrix surfaces associated with the structural features of
the magnetic field, such as null, bald-patch, and minimum points. These
surfaces bound distinct flux systems and accommodate their evolution
through magnetic reconnection. Its utility for understanding the
magnetic configuration of the corona as it relates to space-weather
activity has been demonstrated by many recent works. Here we describe a
new data product: synoptic Q-maps using Helioseismic and Magnetic Imager
(HMI) data since May 2010 as input. For global-scale field, we regularly
compute 2D Q-maps at various heights based on a high-resolution
potential field source surface (PFSS) model. At the upper boundary,
these maps intuitively demonstrate the expansion of coronal holes,
outlining their boundaries formed by streamers and pseudo-streamers. At
lower heights, they additionally reveal an intricate network of closed
(quasi-)separatrix surfaces that can also be involved in the multi-scale
coronal dynamics driven by solar flares, coronal mass ejections, and
solar wind. Future work includes calculations with global MHD models,
using local active region models, and data visualization. The Q-maps
will be available on the SDO/JSOC website and via the Virtual Solar
Observatory.
Title: Tracking Changes in Magnetic Topology in MHD Simulations
Authors: Mikic, Z.; Titov, V. S.; Lionello, R.; Torok, T.; Linker,
J.; Downs, C.
Bibcode: 2016AGUFMSH43B2570M
Altcode:
The topology of the coronal magnetic field plays a key role in the
properties of the corona and the source of the slow solar wind. The
concept of slip-back mapping (Titov et al. 2009) has been applied
to detect open, closed, and disconnected flux systems formed by
reconnection of coronal magnetic fields during a given time interval. In
particular, this technique can identify regions where closed magnetic
field lines became open (e.g., via interchange reconnection), and
conversely, where open field lines became closed. We will describe the
application of this technique to the analysis of 3D MHD simulations
(including those of coronal jets and the propagation of "blobs" in the
solar wind). Research supported by NASA's Living With a Star Program.
Title: The Wide-Field Imager for Solar Probe Plus (WISPR)
Authors: Vourlidas, Angelos; Howard, Russell A.; Plunkett, Simon P.;
Korendyke, Clarence M.; Thernisien, Arnaud F. R.; Wang, Dennis; Rich,
Nathan; Carter, Michael T.; Chua, Damien H.; Socker, Dennis G.; Linton,
Mark G.; Morrill, Jeff S.; Lynch, Sean; Thurn, Adam; Van Duyne, Peter;
Hagood, Robert; Clifford, Greg; Grey, Phares J.; Velli, Marco; Liewer,
Paulett C.; Hall, Jeffrey R.; DeJong, Eric M.; Mikic, Zoran; Rochus,
Pierre; Mazy, Emanuel; Bothmer, Volker; Rodmann, Jens
Bibcode: 2016SSRv..204...83V
Altcode: 2015SSRv..tmp....8V; 2015SSRv..tmp...66B
The Wide-field Imager for Solar PRobe Plus (WISPR) is the sole imager
aboard the Solar Probe Plus (SPP) mission scheduled for launch in
2018. SPP will be a unique mission designed to orbit as close as
7 million km (9.86 solar radii) from Sun center. WISPR employs a
95∘ radial by 58∘ transverse field of view
to image the fine-scale structure of the solar corona, derive the 3D
structure of the large-scale corona, and determine whether a dust-free
zone exists near the Sun. WISPR is the smallest heliospheric imager to
date yet it comprises two nested wide-field telescopes with large-format
(2 K × 2 K) APS CMOS detectors to optimize the performance for their
respective fields of view and to minimize the risk of dust damage,
which may be considerable close to the Sun. The WISPR electronics are
very flexible allowing the collection of individual images at cadences
up to 1 second at perihelion or the summing of multiple images to
increase the signal-to-noise when the spacecraft is further from the
Sun. The dependency of the Thomson scattering emission of the corona
on the imaging geometry dictates that WISPR will be very sensitive
to the emission from plasma close to the spacecraft in contrast to
the situation for imaging from Earth orbit. WISPR will be the first
`local' imager providing a crucial link between the large-scale corona
and the in-situ measurements.
Title: The Contribution of Coronal Jets to the Solar Wind
Authors: Lionello, R.; Török, T.; Titov, V. S.; Leake, J. E.;
Mikić, Z.; Linker, J. A.; Linton, M. G.
Bibcode: 2016ApJ...831L...2L
Altcode: 2016arXiv161003134L
Transient collimated plasma eruptions in the solar corona, commonly
known as coronal (or X-ray) jets, are among the most interesting
manifestations of solar activity. It has been suggested that these
events contribute to the mass and energy content of the corona and
solar wind, but the extent of these contributions remains uncertain. We
have recently modeled the formation and evolution of coronal jets
using a three-dimensional (3D) magnetohydrodynamic (MHD) code with
thermodynamics in a large spherical domain that includes the solar
wind. Our model is coupled to 3D MHD flux-emergence simulations, I.e.,
we use boundary conditions provided by such simulations to drive a
time-dependent coronal evolution. The model includes parametric coronal
heating, radiative losses, and thermal conduction, which enables us to
simulate the dynamics and plasma properties of coronal jets in a more
realistic manner than done so far. Here, we employ these simulations to
calculate the amount of mass and energy transported by coronal jets into
the outer corona and inner heliosphere. Based on observed jet-occurrence
rates, we then estimate the total contribution of coronal jets to the
mass and energy content of the solar wind to (0.4-3.0)% and (0.3-1.0)%,
respectively. Our results are largely consistent with the few previous
rough estimates obtained from observations, supporting the conjecture
that coronal jets provide only a small amount of mass and energy to the
solar wind. We emphasize, however, that more advanced observations and
simulations (including parametric studies) are needed to substantiate
this conjecture.
Title: An Investigation of Time Lag Maps Using Three-dimensional
Simulations of Highly Stratified Heating
Authors: Winebarger, Amy R.; Lionello, Roberto; Downs, Cooper; Mikić,
Zoran; Linker, Jon; Mok, Yung
Bibcode: 2016ApJ...831..172W
Altcode:
The location and frequency of coronal energy release provide a
significant constraint on the coronal heating mechanism. The evolution
of the intensity observed in coronal structures found from time lag
analysis of Atmospheric Imaging Assembly (AIA) data has been used
to argue that heating must occur sporadically. Recently, we have
demonstrated that quasi-steady, highly stratified (footpoint) heating
can produce results qualitatively consistent with the evolution of
observed coronal structures. The goals of this paper are to demonstrate
that time lag analysis of 3D simulations of footpoint heating are
qualitatively consistent with time lag analysis of observations and to
use the 3D simulations to further understand whether time lag analysis
is a useful tool in defining the evolution of coronal structures. We
find the time lag maps generated from simulated data are consistent
with the observed time lag maps. We next investigate several example
points. In some cases, the calculated time lag reflects the evolution of
a unique loop along the line of sight, though there may be additional
evolving structures along the line of sight. We confirm that using
the multi-peak AIA channels can produce time lags that are difficult
to interpret. We suggest using a different high temperature channel,
such as an X-ray channel. Finally, we find that multiple evolving
structures along the line of sight can produce time lags that do not
represent the physical properties of any structure along the line of
sight, although the cross-correlation coefficient of the lightcurves
is high. Considering the projected geometry of the loops may reduce
some of the line-of-sight confusion.
Title: Are Dynamical Sources Essential for the Production of the
Ambient Solar Wind?
Authors: Linker, Jon; Downs, Cooper; Lionello, Roberto; Titov,
Viacheslav; Caplan, Ronald; Riley, Pete; Mikic, Zoran
Bibcode: 2016usc..confE..70L
Altcode:
At a basic level, the large structure of the solar corona and its
connection to the solar wind has been known for many years. In
the classic (near-solar minimum) picture, the slow solar wind is
associated with the streamer belt at low latitudes, while the fast
solar wind arises from coronal holes at higher latitudes. At a deeper
level, important aspects of this connection still puzzle us. One
such controversy is the origin of the slow solar wind. One group of
theories assume that the slow wind primarily arises quasi-statically
from regions of large expansion factor near the boundaries of coronal
holes, while a contrasting set of theories argue that the slow solar
wind is primarily dynamic in origin and involves the reconnection and
exchange of open and closed fields. In this talk, we describe evidence
for both sets of theories, and ongoing and future work that may help
to resolve this question. Work supported by NASA, NSF and AFOSR.
Title: Long-period Intensity Pulsations as the Manifestation of the
Heating Stratification and Timescale in Coronal Loops
Authors: Froment, Clara; Auchère, Frédéric; Aulanier, Guillaume;
Mikić, Zoran; Bocchialini, Karine; Buchlin, Eric; Solomon, Jacques
Bibcode: 2016usc..confE..47F
Altcode:
In solar coronal loops, thermal non-equilibrium (TNE) is a phenomenon
that can occur when the heating is both highly-stratified and
quasi-constant. Unambiguous observational identification of TNE
would thus permit to strongly constrain heating scenarios. Up to
now, while TNE is the standard interpretation of coronal rain, it
was not believed to happen commonly in warm coronal loops. Recently,
the detection of long-period intensity pulsations (periods of several
hours) has been reported with SoHO/EIT. This phenomenon appears to be
very common in loops (Auchère et al. 2014). In Froment et al. 2015,
three intensity-pulsation events studied with SDO/AIA, show strong
evidence for TNE in warm loops. We use realistic loop geometries
from LFFF extrapolations for one of these events are used as input
to a 1D hydrodynamic simulation of TNE. A highly-stratified heating
function is chosen to reproduce the observed period of pulsation and
temperature of the loops. With these conditions, the heating function
has to be asymmetric. The magnetic topology of the LFFF extrapolations
points to the presence of sites of preferred reconnection at one
footpoint, supporting the presence of asymmetric heating. We compared
the properties of the simulated loop with the properties deduced
from observations. We found that the 1D hydrodynamic simulation
can reproduce the large temporal scale intensity properties of the
pulsating loops (Froment et al. 2016, submitted). This simulation
further strengthen the interpretation of the observed pulsations as
signatures of TNE. This implies that the heating for these loops is
highly-stratified and that the frequency of the heating events must
be high compared to the typical cooling time.
Title: The Thermodynamics of Coronal Jets and Their Contribution to
the Solar Wind
Authors: Lionello, Roberto; Török, Tibor; Titov, Viacheslav; Linker,
Jon A.; Mikic, Zoran; James E.; Linton, Mark
Bibcode: 2016usc..confE..11L
Altcode:
Coronal (or X-ray) jets are transient, collimated plasma eruptions
that are observed low in the corona in EUV and soft X-ray bands. It is
widely accepted that they are triggered by reconnection between closed
and open magnetic fields, but their detailed formation mechanisms are
still under debate. Since coronal jets are often seen to extend to
several solar radii, it has been suggested that they may contribute to
powering the solar wind, but the amount of this contribution remains
largely uncertain. Here we present the first MHD simulations of coronal
jets that include the solar wind and a realistic description of the
energy transfer in the corona ("thermodynamic MHD"). The evolution in
our model is driven by the emergence of a magnetic flux rope into an
open magnetic field. We find different types of jets in our simulations,
and discuss their respective formation mechanisms, morphologies, and
emission properties. We also analyze their energy and mass contributions
to the solar wind, and compare them with existing estimations obtained
from observations.
Title: The non-equilibrium ionization modeling of the CME in a 3D
simulated model
Authors: Shen, Chengcai; Raymond, John C.; Mikić, Zoran; Linker,
Jon A.; Reeves, Katharine K.
Bibcode: 2016shin.confE..47S
Altcode:
Non-equilibrium ionization is important in astrophysical environments
where the thermodynamical time scale is shorter than the ionization
or recombination time scales. To understand the observational
properties of the corona mass ejection(CME), it is important to
analyze the non-equilibrium ionization properties. In this work, we
perform post-processing time-dependent ionization calculations for
a three-dimensional CME model developed by Predictive Sciences Inc.,
and present the ionic charge state composition structure derived from
this calculation. Using the plasma temperature, density, and velocity
distributions provide by the MHD simulation, we trace the motion of
plasma in a Lagrangian framework and solve time-dependent ionization
equations. We then construct three-dimensional spatial distributions
of ionic charge states for several chemical elements including carbon,
oxygen, silicon and iron that are typically observed in CME plasma.
Title: Field Line Structure of Separatrix and Qausi-Separatrix
Magnetic Surfaces in the Solar Corona
Authors: Titov, Viacheslav S.; Mikić, Zoran; Downs, Cooper; Török,
Tibor; Lionello, Roberto; Linker, Jon A.
Bibcode: 2016shin.confE.132T
Altcode:
The analysis of the magnetic field topology provides a key framework for
understanding complex phenomena in the solar atmosphere and other cosmic
plasmas where the magnetic field plays an active role. This analysis
is facilitated by the calculation of the so-called squashing factor Q
on the surfaces that bound or cross the magnetic configuration under
study. The Q-factor is a dimensionless quantity that characterizes
the divergence of the field lines on the way between their boundary
end points. For realistic configurations, the Q-maps reveal intricate
networks of high-Q lines, which are, in turn, the cross-sections of
separatrix and quasi-separatrix surfaces present in the magnetic
configuration. The sheer complexity of Q-maps can often be
difficult to interpret. To mitigate this problem, we have developed
a new technique that allows one to efficiently compute the field
line structure of the (quasi-)separatrix surfaces by starting from
their high-Q lines. The underlying algorithm iteratively determines
sets of field-line pairs that bracket null, minimum, and bald-patch
points. Convergence of the algorithm towards the high-Q line on either
side automatically yields approximation of the (quasi-)separatrix
surfaces. We demonstrate the outstanding capabilities of this
technique by reconstructing the magnetic topology for a number of
on-going projects at Predictive Science Inc., which include coronal
mass ejections, streamers, streamer blobs, pseudo-streamers, and
coronal jets. Research supported by NSF/SHINE and NSF/FESD,
and by NASAś HSR and LWS Programs.
Title: Q-Maps: A New Synoptic Data Product for Investigating Dynamic
Coronal Connectivity
Authors: Sun, Xudong; Hoeksema, Jon Todd; Liu, Yang; Mikic, Zoran;
Titov, Viacheslav
Bibcode: 2016SPD....47.1011S
Altcode:
The "squashing degree" Q parameter characterizes the local divergence
of nearby magnetic field lines. It naturally highlights topological
features such as null points, separatrices, and quasi-separatrices where
reconnection preferably takes place. Its utility for understanding the
magnetic configuration of the corona as it relates to space-weather
activity has been demonstrated by many recent works. Here we describe a
new data product: synoptic Q-maps using Helioseismic and Magnetic Imager
(HMI) data since May 2010 as input. For global-scale field, we regularly
compute 2D Q-maps at various heights based on a high-resolution
potential field source surface (PFSS) model. At the upper boundary,
these maps intuitively demonstrate the expansion of coronal holes,
outlining their boundaries formed by streamers and pseudo-streamers. At
lower heights, they additionally reveal an intricate network of closed
(quasi-)separatrix surfaces that can also be involved in the multi-scale
coronal dynamics driven by solar flares, coronal mass ejections, and
solar wind. Future work includes calculations with global MHD models,
using local active region models, and data visualization. The Q-maps
will be available on the SDO/JSOC website and via the Virtual Solar
Observatory.
Title: The Contribution of Jets to Coronal and Solar Wind Energetics:
MHD Simulations
Authors: Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Linker,
Jon A.; Mikic, Zoran; Leake, James E.; Linton, Mark
Bibcode: 2016SPD....4740202L
Altcode:
Transient collimated plasma eruptions in the corona, commonly known as
coronal jets, are among the most interesting manifestations of solar
activity.We use the 3D MHD model with thermodynamics developed at PSI
to investigate the origin, dynamics, and plasma properties of coronal
jets.Our model is coupled with 3D MHD flux emergence simulations,
i.e, we use boundary conditions provided by such simulations to
drive a time-dependent coronal evolution. It includes parametric
coronal heating, radiative losses, and thermal conduction in the
energy equations.This enables us to simulate the energy transfer in
coronal jets in a more realistic manner than done so far and to study
the amount of energy and mass transported by these phenomena into
the higher corona and inner heliosphere. We discuss our results and
compare them with previous estimations obtained from observations.
Title: Modeling Jets in the Corona and Solar Wind
Authors: Torok, Tibor; Lionello, Roberto; Titov, Viacheslav S.; Leake,
James E.; Mikic, Zoran; Linker, Jon A.; Linton, Mark G.
Bibcode: 2016EGUGA..18.2692T
Altcode: 2015arXiv151109350T
Coronal jets are transient, collimated eruptions that occur in
regions of open or semi-open magnetic fields in the solar corona. Our
understanding of these events has significantly improved in recent
years, owing to improved observational capabilities and numerical
simulations. Yet, several important questions concerning coronal jets
remain largely unanswered. For example: What exactly are the physical
mechanisms that heat and accelerate the plasma? And to what extent
do jets contribute to the heating of the corona and in providing
mass and energy to the fast solar wind? Here we present a "new
generation" of coronal-jet simulations that will allow us to address
such questions in more detail than before. In contrast to previous
simulations, our code models the large-scale corona in a spherical
domain, uses an advanced description of the energy transfer in the
corona ("thermodynamic MHD"), and includes the solar wind. As a first
application, we consider a purely radial coronal magnetic field and
a simple coronal heating function that decreases exponentially with
height above the surface. We produce so-called standard and blowout
jets by continuously driving the system at the lower boundary with data
extracted from flux-emergence simulations. We discuss the formation,
dynamics, and evolution of the jets, as well as their contribution to
coronal heating and the solar wind.
Title: Modeling Jets in the Corona and Solar Wind
Authors: Török, T.; Lionello, R.; Titov, V. S.; Leake, J. E.;
Mikić, Z.; Linker, J. A.; Linton, M. G.
Bibcode: 2016ASPC..504..185T
Altcode:
Coronal jets are transient, collimated eruptions that occur in
regions of predominantly open magnetic field in the solar corona. Our
understanding of these events has greatly evolved in recent years but
several open questions, such as the contribution of coronal jets to the
solar wind, remain. Here we present an overview of the observations and
numerical modeling of coronal jets, followed by a brief description of
"next-generation" simulations that include an advanced description
of the energy transfer in the corona ("thermodynamic MHD"), large
spherical computational domains, and the solar wind. These new models
will allow us to address some of the open questions.
Title: Can Large Time Delays Observed in Light Curves of Coronal
Loops Be Explained in Impulsive Heating?
Authors: Lionello, Roberto; Alexander, Caroline E.; Winebarger,
Amy R.; Linker, Jon A.; Mikić, Zoran
Bibcode: 2016ApJ...818..129L
Altcode: 2015arXiv151206146L
The light curves of solar coronal loops often peak first in channels
associated with higher temperatures and then in those associated with
lower temperatures. The delay times between the different narrowband EUV
channels have been measured for many individual loops and recently for
every pixel of an active region observation. The time delays between
channels for an active region exhibit a wide range of values. The
maximum time delay in each channel pair can be quite large, I.e.,
>5000 s. These large time delays make-up 3%-26% (depending on the
channel pair) of the pixels where a trustworthy, positive time delay is
measured. It has been suggested that these time delays can be explained
by simple impulsive heating, I.e., a short burst of energy that heats
the plasma to a high temperature, after which the plasma is allowed to
cool through radiation and conduction back to its original state. In
this paper, we investigate whether the largest observed time delays can
be explained by this hypothesis by simulating a series of coronal loops
with different heating rates, loop lengths, abundances, and geometries
to determine the range of expected time delays between a set of four
EUV channels. We find that impulsive heating cannot address the largest
time delays observed in two of the channel pairs and that the majority
of the large time delays can only be explained by long, expanding
loops with photospheric abundances. Additional observations may rule
out these simulations as an explanation for the long time delays. We
suggest that either the time delays found in this manner may not be
representative of real loop evolution, or that the impulsive heating
and cooling scenario may be too simple to explain the observations,
and other potential heating scenarios must be explored.
Title: A Three-dimensional Model of Active Region 7986: Comparison
of Simulations with Observations
Authors: Mok, Yung; Mikić, Zoran; Lionello, Roberto; Downs, Cooper;
Linker, Jon A.
Bibcode: 2016ApJ...817...15M
Altcode:
In the present study, we use a forward modeling method to construct a 3D
thermal structure encompassing active region 7986 of 1996 August. The
extreme ultraviolet (EUV) emissions are then computed and compared
with observations. The heating mechanism is inspired by a theory on
Alfvén wave turbulence dissipation. The magnetic structure is built
from a Solar and Heliospheric Observatory (SOHO)/MDI magnetogram and
an estimated torsion parameter deduced from observations. We found
that the solution to the equations in some locations is in a thermal
nonequilibrium state. The time variation of the density and temperature
profiles leads to time dependent emissions, which appear as thin,
loop-like structures with uniform cross-section. Their timescale is
consistent with the lifetime of observed coronal loops. The dynamic
nature of the solution also leads to plasma flows that resemble
observed coronal rain. The computed EUV emissions from the coronal
part of the fan loops and the high loops compare favorably with
SOHO/EIT observations in a quantitative comparison. However, the
computed emission from the lower atmosphere is excessive compared to
observations, a symptom common to many models. Some factors for this
discrepancy are suggested, including the use of coronal abundances to
compute the emissions and the neglect of atmospheric opacity effects.
Title: Fast Wave Trains Associated with Solar Eruptions: Insights
from 3D Thermodynamic MHD Simulations
Authors: Downs, C.; Liu, W.; Torok, T.; Linker, J.; Mikic, Z.;
Ofman, L.
Bibcode: 2015AGUFMSH22A..06D
Altcode:
EUV imaging observations during the SDO/AIA era have provided new
insights into a variety of wave phenomena occurring in the low
solar corona. One example is the observation of quasi-periodic,
fast-propagating wave trains that are associated with solar eruptions,
including flares and CMEs. While there has been considerable
progress in understanding such waves from both an observational
and theoretical perspective, it remains a challenge to pin down
their physical origin. In this work, we detail our results from
a case-study 3D thermodynamic MHD simulation of a coronal mass
ejection where quasi-periodic wave trains are generated during the
simulated eruption. We find a direct correlation between the onset of
non-steady reconnection in the flare current sheet and the generation
of quasi-periodic wave train signatures when patchy, collimated
downflows interact with the flare arcade. Via forward modeling of
SDO/AIA observables, we explore how the appearance of the wave trains
is affected by line-of-sight integration and the multi-thermal nature
of the coronal medium. We also examine how the wave trains themselves
are channeled by natural waveguides formed in 3D by the non-uniform
background magnetic field. While the physical association of the
reconnection dynamics to the generation of quasi-periodic wave trains
appears to be a compelling result, unanswered questions posed from
recent observations as well as future prospects will be discussed.
Title: Thermodynamic MHD Simulations of Jets in the Solar Corona
and Inner Heliosphere
Authors: Lionello, R.; Torok, T.; Titov, V. S.; Linker, J.; Mikic,
Z.; Leake, J. E.; Linton, M.
Bibcode: 2015AGUFMSH11F..02L
Altcode:
Coronal jets are transient, collimated plasma ejections that occur
predominantly in coronal holes and are observed in EUV, soft X-ray,
and occasionally in white-light coronagraphs. While these intriguing
phenomena have been studied and modeled for more than two decades, the
details of their formation mechanism(s) are not yet fully understood,
and their potential role for the generation of the fast solar wind
remains largely elusive. Here we present 3D MHD simulations of
coronal jets which are performed in a large computational domain (up
to 20 solar radii) and incorporate the effects of thermal conduction,
radiative cooling, empirical coronal heating, and the solar wind. These
features allow us to model the plasma properties and energy transfer of
coronal jets in a more realistic manner than done so far, and to study
the amount of energy and mass transported by these phenomena into the
higher corona and inner heliosphere. In order to produce a jet,
we consider a simple, purely radial background magnetic field and
slowly introduce a magnetic flux rope into the coronal configuration
by coupling our model to dynamic flux emergence simulations at the
lower boundary of the computational domain. We find two types of jets
in our simulations: a very impulsive event reminiscent of so-called
blowout jets and a slowly developing, more extended event that produces
a long-lasting signature in the corona. We present synthetic satellite
images for both types of events and discuss their respective formation
mechanisms. Our analysis is supported by a detailed investigation of
the magnetic topology for the blowout-type case and of the transport
of energy and plasma into the higher corona and inner heliosphere for
the long-lasting event.
Title: Slip versus Field-Line Mapping in Describing 3D Reconnection
of Coronal Magnetic Fields
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Downs, C.; Lionello,
R.; Linker, J.
Bibcode: 2015AGUFMSH43A2421T
Altcode:
We demonstrate two techniques for describing the structure of the
coronal magnetic field and its evolution due to reconnection in
numerical 3D simulations of the solar corona and CMEs. These techniques
employ two types of mapping of the boundary of the computational
domain on itself. One of them is defined at a given time moment via
connections of the magnetic field lines to their opposite endpoints. The
other mapping, called slip mapping, relates field line endpoints at two
different time moments and allows one to identify the slippage of plasma
elements due to resistivity across field lines for a given time interval
(Titov et al. 2009). The distortion of each of these mappings can be
measured by using the so-called squashing factor Q (Titov 2007). The
high-Q layers computed for the first and second mappings define,
respectively, (quasi-)separatrix surfaces and reconnection fronts in
evolving magnetic configurations. Analyzing these structural features,
we are able to reveal topologically different domains and reconnected
flux systems in the configurations, in particular, open, closed and
disconnected magnetic flux tubes, as well as quantify the related
magnetic flux transfer. Comparison with observations makes it possible
also to relate these features to observed morphological elements
such as flare loops and ribbons, and EUV dimmings. We illustrate
these general techniques by applying them to particular data-driven
MHD simulations. *Research supported by NASA's HSR and LWS Programs,
and NSF/SHINE and NSF/FESD.
Title: How Much Energy Can Be Stored in Solar Active Region Magnetic
Fields?
Authors: Linker, J.; Downs, C.; Torok, T.; Titov, V. S.; Lionello,
R.; Mikic, Z.; Riley, P.
Bibcode: 2015AGUFMSH52A..08L
Altcode:
Major solar eruptions such as X-class flares and very fast coronal
mass ejections usually originate in active regions on the Sun. The
energy that powers these events is believed to be stored as free
magnetic energy (energy above the potential field state) prior to
eruption. While coronal magnetic fields are not in general force-free,
active regions have very strong magnetic fields and at low coronal
heights the plasma beta is therefore very small, making the field (in
equilibrium) essentially force-free. The Aly-Sturrock theorem shows that
the energy of a fully force-free field cannot exceed the energy of the
so-called open field. If the theorem holds, this places an upper limit
on the amount of free energy that can be stored: the maximum free energy
(MFE) is the difference between the open field energy and the potential
field energy of the active region. In thermodynamic MHD simulations of
a major eruption (the July 14, 2000 'Bastille' day event) and a modest
event (February 13, 2009, we have found that the MFE indeed bounds the
energy stored prior to eruption. We compute the MFE for major eruptive
events in cycles 23 and 24 to investigate the maximum amount of energy
that can be stored in solar active regions.Research supported by AFOSR,
NASA, and NSF.
Title: Diagnosing the Properties of the Solar Wind using Magnetic
Topology
Authors: Mikic, Z.; Titov, V. S.; Lionello, R.; Downs, C.; Linker,
J.; Torok, T.; Riley, P.
Bibcode: 2015AGUFMSH31C2436M
Altcode:
Recent work suggests that the topology of the coronal magnetic field
plays a key role in the source and properties of the slow solar wind,
through the collection of separatrix surfaces and quasi-separatrix
layers (QSLs) that define the S-web (Antiochos et al. 2011; Linker et
al. 2011; Titov et al. 2011). We have accumulated extensive experience
with using the squashing factor Q to analyze the underlying structural
skeleton of the coronal magnetic field, to identify magnetic null
points, separator field lines, QSLs, and separatrix surfaces, and their
relationship with the topology of coronal hole boundaries. This will
be extended by implementing slip mapping (Titov et al. 2009) to detect
open, closed, and disconnected flux systems that are formed due to
magnetic reconnection in a coronal model driven by both the differential
rotation and evolution of the photospheric magnetic field. This idea
is based on using forward and backward differences in time between
the field line mapping expected from ideal MHD motions and the actual
mapping to diagnose magnetic reconnection. This technique can identify
regions in the photosphere where closed magnetic field lines are about
to open (e.g., via interchange reconnection), and conversely, where open
field lines are about to close. We will use these concepts to develop
tools that relate the changing magnetic topology to the properties of
the solar wind, to plan and interpret Solar Probe Plus and Solar Orbiter
observations. Research supported by NASA's Living With a Star Program.
Title: Particle Acceleration at Low Coronal Compression Regions
and Shocks
Authors: Schwadron, N. A.; Lee, M. A.; Gorby, M.; Lugaz, N.; Spence,
H. E.; Desai, M.; Török, T.; Downs, C.; Linker, J.; Lionello,
R.; Mikić, Z.; Riley, P.; Giacalone, J.; Jokipii, J. R.; Kota, J.;
Kozarev, K.
Bibcode: 2015ApJ...810...97S
Altcode:
We present a study on particle acceleration in the low corona
associated with the expansion and acceleration of coronal mass ejections
(CMEs). Because CME expansion regions low in the corona are effective
accelerators over a finite spatial region, we show that there is a
rigidity regime where particles effectively diffuse away and escape
from the acceleration sites using analytic solutions to the Parker
transport equation. This leads to the formation of broken power-law
distributions. Based on our analytic solutions, we find a natural
ordering of the break energy and second power-law slope (above the
break energy) as a function of the scattering characteristics. These
relations provide testable predictions for the particle acceleration
from low in the corona. Our initial analysis of solar energetic particle
observations suggests a range of shock compression ratios and rigidity
dependencies that give rise to the solar energetic particle (SEP)
events studied. The wide range of characteristics inferred suggests
competing mechanisms at work in SEP acceleration. Thus, CME expansion
and acceleration in the low corona may naturally give rise to rapid
particle acceleration and broken power-law distributions in large
SEP events.
Title: A Method for Localization of the Photospheric Tangential
Electric Field within Flux-Emergence Regions
Authors: Titov, Viacheslav S.; Caplan, Ronald M.; Downs, Cooper;
Mikic, Zoran; Linker, Jon
Bibcode: 2015shin.confE..38T
Altcode:
Data-driven MHD simulations of evolving coronal magnetic fields
require the knowledge of the electric field at the photospheric
boundary. A time sequence of the normal component of the photospheric
magnetic field derived from observations allows one to determine only
a solenoidal component of the tangential electric field, while its
potential component remains free and should be constrained from other
conditions. It is not difficult to show that the simplest approach
of setting this potential component to zero yields the tangential
electric field with unphysical asymptotics at large distances from
the regions of emergence/submergence of the photospheric magnetic
field. The resulting tangential electric field decays as the inverse
square of the distance leading to the appearance of artificial boundary
layers at the photosphere in the simulated MHD evolution. To mitigate
this problem, we propose a method that enables one to obtain the total
tangential electric field as localized as needed while still having
the right solenoidal component constrained from the observed time
sequence of magnetograms. We demonstrate that the required electric
field is expressed in terms of solutions to the stationary 2D heat
equation with a spatially-dependent thermal conductivity. The form
of this dependence can be chosen so as to properly localize the
tangential electric field. The proposed method helps to produce
physically consistent boundary conditions for MHD modeling of the
evolving coronal magnetic configurations.
Title: How Much Energy Can Be Stored in Active Region Magnetic Fields?
Authors: Linker, Jon A.; Torok, Tibor; Downs, Cooper; Titov,
Viacheslav; Lionello, Roberto; Riley, Pete; Mikic, Zoran
Bibcode: 2015shin.confE..77L
Altcode:
Major solar eruptions such as X-class flares and very fast coronal
mass ejections usually originate in active regions on the Sun. The
energy that powers these events is believed to be stored as free
magnetic energy (energy above the potential field state) prior to
eruption. While coronal magnetic fields are not in general force-free,
active regions have very strong magnetic fields and at low coronal
heights the plasma beta is very small, making the field (in equilibrium)
essentially force-free. The Aly-Sturrock theorem shows that the energy
of a force-free field cannot exceed the energy of the so-called open
field. If the theorem holds, this places an upper limit on the amount
of free energy that can be stored. We investigate the magnetic energy
storage and release in full thermodynamic MHD simulations of a major
event (the July 14, 2000 'Bastille' day event) and a modest event
(February 13, 2009) and relate it to the potential and open field
energies for these active regions. We discuss the usefulness of the
open field energy as a guide to how much energy can be stored in an
active region.
Title: Thermodynamic 3D MHD Modeling of Coronal Jets
Authors: Lionello, Roberto; Torok, Tibor; Titov, Viacheslav S.; Leake,
James E.; Linton, Mark G.; Linker, Jon A.; Mikic, Zoran
Bibcode: 2015shin.confE..32L
Altcode:
Transient collimated plasma eruptions in the corona, so-called
'standard' and 'blowout' coronal jets, are among the most intriguing
manifestations of solar activity. We use the PSI 'thermodynamic'
3D MHD model to improve our understanding of the origin, dynamics,
and plasma properties of coronal jets. Our code models the corona by
taking into account thermal conduction, radiative cooling, empirical
coronal heating, and the solar wind. These properties enable us to
simulate the energy transfer in coronal jets in a more realistic manner
than done so far, and to study the amount of energy and mass transported
by these phenomena into the higher corona and solar wind. Here we couple
our model with 3D MHD flux emergence simulations, i.e, we use boundary
conditions provided by such simulations to drive a time-dependent
coronal evolution. In particular, we study the topological properties
of the magnetic fields associated with jets, how the jet appears in
EUV and soft X-ray emission, and its signature in the inner heliosphere.
Title: Connecting the evolution and properties of CMEs to their low
coronal signatures. A modeling case study of the ‘simple’ Feb
13 2009 event
Authors: Downs, Cooper; Török, Tibor; Titov, Viacheslav; Liu, Wei;
Linker, Jon; Mikić, Zoran
Bibcode: 2015TESS....130401D
Altcode:
The early onset and and evolution of a CME is a process that features
an intimate coupling between the erupting flux-system and the
ambient corona. For this reason low coronal signatures that we often
observe in the EUV can be used to infer information on the physical
nature and evolution of CMEs. In this presentation we will discuss
a 3D thermodynamic MHD simulation of the Feb 13 2009 eruption,
which occurred from an isolated region during solar minimum and
produced well characterized EUV wave and transient coronal dimming
features. Using observations as a guide, we simulate the entire
evolution of the eruption and global corona, starting from the initial
stable configuration through onset and evolution to the post-eruptive
reconfiguration. With a particular focus on coronal dimmings, we
track how the connectivity of the erupting flux-rope evolves with
time and how this relates to corresponding dimmings in synthetic EUV
observables. We find that the appearance of the core dimming regions
and their migration over time can be related to when and where the
erupting rope reconnects with itself and the adjacent arcade. Other
aspects related to CME evolution, such as the generation of an EUV
wave and quasi-periodic fast-propagating waves are also discussed.
Title: The Role of GONG observations in Global MHD Modeling
Authors: Linker, Jon; Downs, Cooper; Lionello, Roberto; Caplan,
Ronald M.; Riley, Pete; Mikić, Zoran; Arge, Nick; Henney, Carl
Bibcode: 2015TESS....121402L
Altcode:
The solar magnetic field is an essential aspect of any predictive
model of the solar corona. For many years, the magnetic field has been
measured most reliably in the photosphere. So-called ``synoptic''
maps of the photospheric field (actually built up from magnetograms
acquired over the course of the solar rotation) are or have been
available from a number of ground- and space-based observatories,
including the Global Oscillation Network Group (GONG). MHD models of
the solar corona have typically used these maps to develop boundary
conditions. GONG data is unique among the ground-based observatories
in (1) providing magnetogams at a high-cadence (2) providing 24 hour
coverage and (3) supplying helioseismic data that can be used to provide
estimates of new active regions that have emerged on the far side of
the Sun. These three elements are especially important as MHD models
attempt to address the time-dependent nature of the corona. In this
talk we describe how the combination of flux transport models driven by
GONG data, along with estimates of far side active region emergence,
can be used in coronal MHD modeling and the advantages gained from
this approach. Work supported by AFOSR, NASA, and NSF.
Title: Non-equilibrium Ionization Modeling of Simulated
Pseudostreamers in a Solar Corona Model
Authors: Shen, Chengcai; Raymond, John C.; Mikić, Zoran; Linker,
Jon; Reeves, Katharine K.; Murphy, Nicholas A.
Bibcode: 2015TESS....140903S
Altcode:
Time-dependent ionization is important for diagnostics of coronal
streamers, where the thermodynamic time scale could be shorter than
the ionization or recombination time scales, and ions are therefor
in non-equilibrium ionization states. In this work, we perform
post-processing time-dependent ionization calculations for a three
dimensional solar corona and inner heliosphere model from Predictive
Sciences Inc. (Mikić & Linker 1999) to analyze the influence of
non-equilibrium ionization on emission from coronal streamers. Using the
plasma temperature, density, velocity and magnetic field distributions
provided by the 3D MHD simulation covering the Whole Sun Month
(Carrington rotation CR1913, 1996 August 22 to September 18), we
calculate non-equilibrium ionization states in the region around a
pseudostreamer. We then obtain the synthetic emissivities with the
non-equilibrium ion populations. Under the assumption that the corona is
optically thin, we also obtain intensity profiles of several emission
lines. We compare our calculations with intensities of Lyman-alpha
lines and OVI lines from SOHO/Ultraviolet Coronagraph Spectrometer
(UVCS) observations at 14 different heights. The results show that
intensity profiles of both Lyman-alpha and OVI lines match well UVCS
observations at low heights. At large heights, OVI intensites are higher
for non-equilibrium ionization than equilibrium ionization inside this
pseudostreamer. The assumption of ionization equilibrium would lead to
a underestimate of the OVI intensity by about ten percent at a height
of 2 solar radii, and the difference between these two ionization
cases increases with height. The intensity ratio of OVI 1032 line to
OVI 1037 lines is also obtained for non-equilibrium ionization modeling.
Title: Magnetic Topology of the Global MHD Configuration on 2010
August 1-2
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J.; Panasenco, O.
Bibcode: 2014AGUFMSH23A4148T
Altcode:
It appears that the global magnetic topology of the solar corona
predetermines to a large extent the magnetic flux transfer during
solar eruptions. We have recently analyzed the global topology for
a source-surface model of the background magnetic field at the time
of the 2010 August 1-2 sympathetic CMEs (Titov et al. 2012). Now we
extend this analysis to a more accurate thermodynamic MHD model of
the solar corona. As for the source-surface model, we find a similar
triplet of pseudo-streamers in the source regions of the eruptions. The
new study confirms that all these pseudo-streamers contain separatrix
curtains that fan out from a basic magnetic null point, individual
for each of the pseudo-streamers. In combination with the associated
separatrix domes, these separatrix curtains fully isolate adjacent
coronal holes of the like polarity from each other. However, the size
and shape of the coronal holes, as well as their open magnetic fluxes
and the fluxes in the lobes of the separatrix domes, are very different
for the two models. The definition of the open separator field lines,
where the (interchange) reconnection between open and closed magnetic
flux takes place, is also modified, since the structurally unstable
source-surface null lines do not exist anymore in the MHD model. In
spite of all these differences, we reassert our earlier hypothesis
that magnetic reconnection at these nulls and the associated separators
likely plays a key role in coupling the successive eruptions observed
by SDO and STEREO. The results obtained provide further validation of
our recent simplified MHD model of sympathetic eruptions (Török et
al. 2011). Research supported by NASA's Heliophysics Theory and LWS
Programs, and NSF/SHINE and NSF/FESD.
Title: Application of a Solar Wind Model Driven by Turbulence
Dissipation to a 2D Magnetic Field Configuration
Authors: Lionello, Roberto; Velli, Marco; Downs, Cooper; Linker,
Jon A.; Mikić, Zoran
Bibcode: 2014ApJ...796..111L
Altcode: 2014arXiv1410.1789L
Although it is widely accepted that photospheric motions provide
the energy source and that the magnetic field must play a key role
in the process, the detailed mechanisms responsible for heating
the Sun's corona and accelerating the solar wind are still not
fully understood. Cranmer et al. developed a sophisticated,
one-dimensional (1D), time-steady model of the solar wind with
turbulence dissipation. By varying the coronal magnetic field, they
obtain, for a single choice of wave properties, a realistic range
of slow and fast wind conditions with a sharp latitudinal transition
between the two streams. Using a 1D, time-dependent model of the solar
wind of Lionello et al., which incorporates turbulent dissipation
of Alfvén waves to provide heating and acceleration of the plasma,
we have explored a similar configuration, obtaining qualitatively
equivalent results. However, our calculations suggest that the rapid
transition between slow and fast wind suggested by this 1D model may
be disrupted in multidimensional MHD simulations by the requirement
of transverse force balance.
Title: Coronal Modeling with Flux-Evolved Maps: Comparison with
Observations
Authors: Linker, J.; Downs, C.; Lionello, R.; Caplan, R. M.; Mikic,
Z.; Riley, P.; Henney, C. J.; Arge, C. N.
Bibcode: 2014AGUFMSH23C..02L
Altcode:
MHD simulations of the solar corona rely on maps of the solar magnetic
field for input as boundary conditions. These "synoptic" maps (available
from a number of ground-based and space-based solar observatories) are
built up over a solar rotation. A well-known problem with this approach
is that the maps contain data that is as much as 27 days old. The
Sun's magnetic flux is always evolving, and these changes in the flux
affect coronal and heliospheric structure. Flux evolution models can
in principle provide a more accurate specification, by estimating
the likely state of the photospheric magnetic field on unobserved
portions of the Sun. The Air Force Data Assimilative Photospheric
flux Transport (ADAPT) model (Arge et al. 2010) is especially well
suited for this purpose. ADAPT can also incorporate information from
helioseismic acoustic images to estimate the emergence of new active
regions on the Sun's far side. In this presentation we describe MHD
models with boundary conditions derived from ADAPT maps. We investigate
the June-August 2010 time period, when there was significant coronal
hole evolution observed by the STEREO and SDO spacecraft. We compare
model results using ADAPT maps, including those with far side data,
as well as models using traditional synoptic maps, to STEREO EUVI and
SDO AIA data. Research supported by AFOSR & NASA.
Title: Towards a Thermodynamic 3D MHD Model of Coronal Jets
Authors: Lionello, R.; Torok, T.; Linker, J.; Mikic, Z.
Bibcode: 2014AGUFMSH53D..06L
Altcode:
Transient collimated plasma eruptions in the corona, so-called
"standard" and "blowout" coronal jets, are among the most intriguing
manifestations of solar activity. We have begun to use the PSI
"thermodynamic" 3D MHD model to improve our understanding of the
origin, dynamics, and plasma properties of coronal jets. Our code
models the corona by taking into account thermal conduction, radiative
cooling, empirical coronal heating, and the solar wind, and it is
capable of using observed magnetograms as boundary condition for the
magnetic field. Furthermore, the model is coupled with 3D MHD flux
emergence simulations, i.e it can use boundary conditions provided by
such simulations to drive a time-dependent coronal evolution. These
properties enable us to simulate the energy transfer in coronal jetsin
a more realistic manner. We will present preliminary results.
Title: Particle Acceleration in the Low Corona Over Broad Longitudes:
Coupling MHD and 3D Particle Simulations
Authors: Gorby, M.; Schwadron, N.; Torok, T.; Downs, C.; Lionello,
R.; Linker, J.; Titov, V. S.; Mikic, Z.; Riley, P.; Desai, M. I.;
Dayeh, M. A.
Bibcode: 2014AGUFMSH21B4127G
Altcode:
Recent work on the coupling between the Energetic Particle Radiation
Environment Module (EPREM, a 3D energetic particle model) and
Magnetohydrodynamics Around a Sphere (MAS, an MHD code developed
at Predictive Science, Inc.) has demonstrated the efficacy of
compression regions around fast coronal mass ejections (CMEs) for
particle acceleration low in the corona (∼ 3 - 6 solar radii). These
couplings show rapid particle acceleration over a broad longitudinal
extent (∼ 80 degrees) resulting from the pile-up of magnetic flux in
the compression regions and their subsequent expansion. The challenge
for forming large SEP events in such compression-acceleration scenarios
is to have enhanced scattering within the acceleration region while
also allowing for efficient escape of accelerated particles downstream
(away from the Sun) from the compression region. We present here
the most recent simulation results including energetic particle and
CME plasma profiles, the subsequent flux and dosages at 1AU, and an
analysis of the compressional regions as efficient accelerators.
Title: Characterizing a Model of Coronal Heating and Solar Wind
Acceleration Based on Wave Turbulence.
Authors: Downs, C.; Lionello, R.; Mikic, Z.; Linker, J.; Velli, M.
Bibcode: 2014AGUFMSH31B..04D
Altcode:
Understanding the nature of coronal heating and solar wind acceleration
is a key goal in solar and heliospheric research. While there have
been many theoretical advances in both topics, including suggestions
that they may be intimately related, the inherent scale coupling
and complexity of these phenomena limits our ability to construct
models that test them on a fundamental level for realistic solar
conditions. At the same time, there is an ever increasing impetus to
improve our spaceweather models, and incorporating treatments for
these processes that capture their basic features while remaining
tractable is an important goal. With this in mind, I will give an
overview of our exploration of a wave-turbulence driven (WTD) model for
coronal heating and solar wind acceleration based on low-frequency
Alfvénic turbulence. Here we attempt to bridge the gap between
theory and practical modeling by exploring this model in 1D HD and
multi-dimensional MHD contexts. The key questions that we explore
are: What properties must the model possess to be a viable model for
coronal heating? What is the influence of the magnetic field topology
(open, closed, rapidly expanding)? And can we simultaneously capture
coronal heating and solar wind acceleration with such a quasi-steady
formulation? Our initial results suggest that a WTD based formulation
performs adequately for a variety of solar and heliospheric conditions,
while significantly reducing the number of free parameters when
compared to empirical heating and solar wind models. The challenges,
applications, and future prospects of this type of approach will also
be discussed.
Title: Verification of Coronal Loop Diagnostics Using Realistic
Three-dimensional Hydrodynamic Models
Authors: Winebarger, Amy R.; Lionello, Roberto; Mok, Yung; Linker,
Jon A.; Mikić, Zoran
Bibcode: 2014ApJ...795..138W
Altcode:
Many different techniques have been used to characterize the plasma
in the solar corona: density-sensitive spectral line ratios are
used to infer the density, the evolution of coronal structures in
different passbands is used to infer the temperature evolution,
and the simultaneous intensities measured in multiple passbands
are used to determine the emission measure distributions. All these
analysis techniques assume that the intensity of the structures can
be isolated through background subtraction. In this paper, we use
simulated observations from a three-dimensional hydrodynamic simulation
of a coronal active region to verify these diagnostics. The density
and temperature from the simulation are used to generate images in
several passbands and spectral lines. We identify loop structures in
the simulated images and calculate the background. We then determine the
density, temperature, and emission measure distribution as a function of
time from the observations and compare these with the true temperature
and density of the loop. We find that the overall characteristics
of the temperature, density, and emission measure are recovered by
the analysis methods, but the details are not. For instance, the
emission measure curves calculated from the simulated observations
are much broader than the true emission measure distribution, though
the average temperature evolution is similar. These differences are
due, in part, to a limitation of the analysis methods, but also to
inadequate background subtraction.
Title: 3D Coronal Density Reconstruction and Retrieving the Magnetic
Field Structure during Solar Minimum
Authors: Kramar, M.; Airapetian, V.; Mikić, Z.; Davila, J.
Bibcode: 2014SoPh..289.2927K
Altcode: 2014SoPh..tmp...63K; 2014arXiv1405.0951K
Measurement of the coronal magnetic field is a crucial ingredient in
understanding the nature of solar coronal phenomena at all scales. We
employed STEREO/COR1 data obtained during a deep minimum of solar
activity in February 2008 (Carrington Rotation CR 2066) to retrieve
and analyze the three-dimensional (3D) coronal electron density in
the range of heights from 1.5 to 4 R⊙ using a tomography
method. With this, we qualitatively deduced structures of the coronal
magnetic field. The 3D electron-density analysis is complemented
by the 3D STEREO/EUVI emissivity in the 195 Å band obtained by
tomography for the same CR. A global 3D MHD model of the solar corona
was used to relate the reconstructed 3D density and emissivity to
open/closed magnetic-field structures. We show that the density-maximum
locations can serve as an indicator of current-sheet position, while the
locations of the density-gradient maximum can be a reliable indicator of
coronal-hole boundaries. We find that the magnetic-field configuration
during CR 2066 has a tendency to become radially open at heliocentric
distances greater than 2.5 R⊙. We also find that the
potential-field model with a fixed source surface is inconsistent with
the boundaries between the regions with open and closed magnetic-field
structures. This indicates that the assumption of the potential nature
of the coronal global magnetic field is not satisfied even during the
deep solar minimum. Results of our 3D density reconstruction will help
to constrain solar coronal-field models and test the accuracy of the
magnetic-field approximations for coronal modeling.
Title: A Method for Embedding Circular Force-free Flux Ropes in
Potential Magnetic Fields
Authors: Titov, V. S.; Török, T.; Mikic, Z.; Linker, J. A.
Bibcode: 2014ApJ...790..163T
Altcode:
We propose a method for constructing approximate force-free equilibria
in pre-eruptive configurations in which a thin force-free flux rope is
embedded into a locally bipolar-type potential magnetic field. The flux
rope is assumed to have a circular-arc axis, a circular cross-section,
and electric current that is either concentrated in a thin layer at the
boundary of the rope or smoothly distributed across it with a maximum
of the current density at the center. The entire solution is described
in terms of the magnetic vector potential in order to facilitate
the implementation of the method in numerical magnetohydrodynamic
(MHD) codes that evolve the vector potential rather than the magnetic
field itself. The parameters of the flux rope can be chosen so that
its subsequent MHD relaxation under photospheric line-tied boundary
conditions leads to nearly exact numerical equilibria. To show the
capabilities of our method, we apply it to several cases with different
ambient magnetic fields and internal flux-rope structures. These
examples demonstrate that the proposed method is a useful tool for
initializing data-driven simulations of solar eruptions.
Title: Time-Dependent MHD Modeling and Coronal Hole Evolution
Authors: Linker, Jon A.; Lionello, Roberto; Downs, Cooper; Caplan,
Ron; Mikic, Zoran; Riley, Pete; Henney, Carl; Arge, Charles N.
Bibcode: 2014shin.confE..25L
Altcode:
MHD simulations of the solar corona based on maps of the solar
magnetic field have been demonstrated to describe many aspects of
coronal structure. However, these models are typically integrated to
steady state, using synoptic or daily-updated magnetic maps to derive
the boundary conditions. The Sun's magnetic flux is always evolving,
and these changes in the flux affect the structure and dynamics of
the corona and heliosphere. In this presentation, we describe an
approach to evolutionary models of the corona and solar wind, using
time-dependent boundary conditions. A key aspect of our approach is
the use of the Air Force Data Assimilative Photospheric flux Transport
(ADAPT) model to develop time-evolving boundary conditions for the
magnetic field. In this study, we use a simplified (zero-beta) MHD
model to investigate coronal hole evolution during the June-August
2010 time period (Carrington rotations 2098-2099). We compare modeled
and observed coronal holes, and discuss implications of coronal hole
evolution for the origin of the slow solar wind. We discuss some of the
difficulties that arise in creating a time-dependent boundary map using
the limited assimilation window available from an earth-based view.
Title: Verification of Loop Diagnostics
Authors: Winebarger, Amy R.; Lionello, Roberto; Mok, Yung; Linker,
Jon A.; Mikic, Zoran
Bibcode: 2014AAS...22431203W
Altcode:
Many different techniques have been used to characterize the plasma
in the solar corona: density-sensitive spectral line ratios are
used to infer the density, the evolution of coronal structures in
different passbands is used to infer the temperature evolution,
and the simultaneous intensities measured in multiple passbands are
used to determine the emission measure. All these analysis techniques
assume that the intensity of the structures can be isolated through
background subtraction. In this paper, we use simulated observations
from a 3D hydrodynamic simulation of a coronal active region to verify
these diagnostics. The density and temperature from the simulation are
used to generate images in several passbands and spectral lines. We
identify loop structures in the simulated images and calculate the loop
background. We then determine the density, temperature and emission
measure distribution as a function of time from the observations
and compare with the true temperature and density of the loop. We
find that the overall characteristics of the temperature, density,
and emission measure are recovered by the analysis methods, but the
details of the true temperature and density are not. For instance, the
emission measure curves calculated from the simulated observations are
much broader than the true emission measure distribution, though the
average temperature evolution is similar. These differences are due,
in part, to inadequate background subtraction, but also indicate a
limitation of the analysis methods.
Title: Thermal Non-equilibrium Consistent with Widespread Cooling
Authors: Winebarger, Amy R.; Lionello, Roberto; Mikic, Zoran; Linker,
Jon A.; Mok, Yung
Bibcode: 2014AAS...22432341W
Altcode:
Time correlation analysis has been used to show widespread cooling
in the solar corona; this cooling has been interpreted as a result of
impulsive (nanoflare) heating. In this work, we investigate wide-spread
cooling using a 3D model for a solar active region which has been
heated with highly stratified heating. This type of heating drives
thermal non-equilibrium solutions, meaning that though the heating is
effectively steady, the density and temperature in the solution are
not. We simulate the expected observations in narrowband EUV images
and apply the time correlation analysis. We find that the results
of this analysis are qualitatively similar to the observed data. We
discuss additional diagnostics that may be applied to differentiate
between these two heating scenarios.
Title: MHD Modeling of Coronal Hole Evolution
Authors: Linker, Jon A.; Lionello, Roberto; Downs, Cooper; Caplan,
Ron; Mikic, Zoran; Riley, Pete; Henney, Carl John; Arge, Charles
Bibcode: 2014AAS...22432336L
Altcode:
MHD simulations of the solar corona based on maps of the solar
magnetic field have been demonstrated to describe many aspects of
coronal structure. However, these models are typically integrated to
steady state, using synoptic or daily-updated magnetic maps to derive
the boundary conditions. The Sun's magnetic flux is always evolving,
and these changes in the flux affect the structure and dynamics of
the corona and heliosphere. In this presentation, we describe an
approach to evolutionary models of the corona and solar wind, using
time-dependent boundary conditions. A key aspect of our approach is
the use of the Air Force Data Assimilative Photospheric flux Transport
(ADAPT) model to develop time-evolving boundary conditions for the
magnetic field. In this study, we use a simplified (zero-beta) MHD
model to investigate coronal hole evolution during the June-August
2010 time period (Carrington rotations 2098-2099). We compare modeled
and observed coronal holes, and discuss implications of coronal hole
evolution for the origin of the slow solar wind.
Title: Developing 3D CME Models
Authors: Mikic, Zoran; Torok, Tibor; Titov, Viacheslav; Linker,
Jon A.; Reeves, Kathy
Bibcode: 2014AAS...22421808M
Altcode:
We describe the development of CME models in three dimensions,
including the energization of active regions and the initiation
of eruptions via flux cancellation. We contrast the dynamics from
idealized zero-beta models with more sophisticated models based on
thermodynamic solutions. We explore the effect of the strength of the
magnetic field in the active region (or, more appropriately, the amount
of smoothing applied to the observed magnetic field), the profiles
for transverse field emergence or applied shear, and the nature of
the flux cancellation, on the dynamics of eruptions. In particular,
our interest is in understanding which effects lead to fast CMEs.
Title: 3D Coronal Density Reconstruction and Retrieving the Magnetic
Field Structures during Solar Minimum and Maximum
Authors: Kramar, Maxim; Airapetian, Vladimir; Mikic, Zoran
Bibcode: 2014shin.confE.101K
Altcode:
Measurement of the coronal magnetic field is a crucial ingredient in
understanding the nature of solar coronal phenomena at all scales. We
employ STEREO/COR1 data obtained during minimum and maximum of solar
activity (Carrington rotations, CR 2066 and 2112) to retrieve and
analyze the three-dimensional (3D) coronal electron density in the
range of heights from 1.5 to 4 Rsun using a tomography method and
qualitatively deduce structures of the coronal magnetic field. The
3D electron density analysis is complemented by the 3D STEREO/EUVI
emissivity in 195 A band obtained by tomography for the same CR
periods. A global 3D MHD model of the solar corona was used to
relate the reconstructed 3D density and emissivity to open/closed
magnetic field structures. We show that the locations of density
maximum can serve as an indicator of current sheet position, while
the locations of the maximum of the density gradient can be a reliable
indicator of coronal hole boundaries. We find that the magnetic field
configuration during CR 2066 has a tendency to become radially open
at heliocentric distances greater than 2.5 Rsun. We also find that
the potential field model with a fixed source surface (PFSS) is not
consistent with the positions of the boundaries between the regions
with open and closed magnetic field structures. This indicates that
the assumption of the potential nature of the coronal global magnetic
field is not satisfied even during the deep solar minimum. Results
of our 3D density reconstruction will help to constrain solar coronal
field models and test the accuracy of the magnetic field approximations
for coronal modeling.
Title: Computing EUV and Soft X-rays Emissions from an Active Region
in 3D
Authors: Mok, Yung; Lionello, Roberto; Mikic, Zoran; Linker, Jon A.
Bibcode: 2014AAS...22432340M
Altcode:
EUV and soft X-rays can be a useful tool for plasma diagnostics when
applied to an active region. Instead of trying to unfold the plasma
properties from the observed emissions, we use a forward modeling
method on AR 7986. Namely, we first compute the thermal structure of the
active region. The resulting density and temperature profiles are then
used to compute the expected emissions, and the synthetic images are
compared with observations. Our model reproduces many features of an
observed active region, including a dark neutral line, coronal loops,
fan loops, etc. The time evolution of the emissions show plasma flows
that resemble coronal rains. Quantitatively, the coronal part of the
loops compare favorably with observations on all three coronal lines of
EIT, although their footpoints are somewhat brighter than observed. The
discrepancy may be attributed to the inaccurate abundances used to
compared the emissions in the low corona and below.Work supported by
the Heliophysics Theory Program of NASA.
Title: Characterizing a Closed Field Coronal Heating Model Inspired
by Wave Turbulence
Authors: Downs, Cooper; Lionello, Roberto; Mikić, Zoran; Linker,
Jon A.; Velli, Marco
Bibcode: 2014shin.confE.156D
Altcode:
To simulate the energy balance of coronal plasmas on macroscopic scales,
we often require the specification of the coronal heating mechanism
in some functional form. To go beyond empirical formulations and to
build a more physically motivated heating function, we investigate
the wave-turbulence driven (WTD) phenomenology for the heating of
closed coronal loops. To do so, we employ an implementation of non-WKB
equations designed to capture the large-scale propagation, reflection,
and dissipation of wave turbulence along a loop. The parameter space
of this model is explored by solving the coupled WTD and hydrodynamic
equations in 1D for an idealized loop, and the relevance to a range
of solar conditions is established by computing solutions for several
hundred loops extracted from a realistic 3D coronal field. Due to
the implicit dependence of the WTD heating model on loop geometry and
plasma properties along the loop and at the footpoints, we find that
this model can significantly reduce the number of free parameters when
compared to traditional empirical heating models, and still robustly
describe a broad range of quiet-sun and active region conditions. The
importance of the self-reflection term in producing realistic heating
scale heights and thermal non-equilibrium cycles is discussed, which
has relevance to the heating and cooling signatures often observed in
active region cores.
Title: Application of a Solar Wind Model Driven by Turbulence
Dissipation to a 2D Magnetic Field Configuration
Authors: Lionello, Roberto; Velli, Marco; Downs, Cooper; Linker,
Jon A.; Mikic, Zoran
Bibcode: 2014shin.confE.141L
Altcode:
Solar physicists are still actively investigating the mechanisms
responsible for heating the Sun's corona and accelerating the solar
wind, although it is widely accepted that photospheric motions provide
the energy source and that the magnetic field must play a key role in
the process. Cranmer et al 2007 developed a sophisticated, 1D,
time-steady model of the solar wind with turbulence dissipation. By
varying the coronal magnetic field, they obtain, for a single choice
of wave properties, a realistic range of slow and fast wind conditions
with a sharp latitudinal transition between the two streams. Using
the 1D, time-dependent model of the solar wind of Cranmer et al. 2007,
which incorporates turbulent dissipation of Alfvén waves to provide
heating and acceleration of the plasma, we have explored a similar
configuration, obtaining qualitatively equivalent results. However,
we suspect that the bifurcation between slow and fast wind suggested
by this 1D model may not occur in multidimensional MHD simulations.
Title: Non-Equilibrium Ionization Calculations in MHD Simulations
During a Whole Sun Month
Authors: Shen, Chengcai; Raymond, John C.; Mikić, Zoran; Linker,
Jon A.; Murphy, Nicholas A.; Reeves, Katharine K.
Bibcode: 2014shin.confE..99S
Altcode:
Time-dependent ionization is important in astrophysical environments
where the thermodynamic time scale is shorter than ionization
time scale. In this work, we develop a set of FORTRAN routines to
perform fast non-equilibrium ionization calculations based on MHD
simulations. Using MHD simulation results, we trace the movements of
plasma in a Lagrangian framework, and obtain evolutionary history
of temperature and electron density. The time-dependent ionization
equations are solved using the eigenvalue method, and an adaptive
time-step strategy is used to improve the computational efficiency. We
then perform non-equilibrium ionization calculations for a thermodynamic
MHD model during Carrington rotation 1913 reported by Mikić et al., and
obtain the ion charge composition. We also calculate the intensities
of UV lines measured by the Ultraviolet Coronagraph Spectrometer
(UVCS) on the Solar and Heliospheric Observatory and compare them
with observations.
Title: Distribution of electric currents in source regions of solar
eruptions
Authors: Torok, Tibor; Leake, James E.; Titov, Viacheslav; Archontis,
Vasilis; Mikic, Zoran; Linton, Mark; Dalmasse, Kevin; Aulanier,
Guillaume; Kliem, Bernhard
Bibcode: 2014AAS...22431202T
Altcode:
There has been a long-lasting debate on the question of whether or
not electric currents in the source regions of solar eruptions are
neutralized. That is, whether or not the direct coronal currents
connecting the photospheric polarities in such regions are surrounded
by return currents of equal amount and opposite direction. In order to
address this question, we consider several mechanisms of source region
formation (flux emergence, photospheric shearing/twisting flows,
and flux cancellation) and quantify the evolution of the electric
currents, using 3D MHD simulations. For the experiments conducted so
far, we find a clear dominance of the direct currents over the return
currents in all cases in which the models produce significant magnetic
shear along the source region's polarity inversion line. This suggests
that pre-eruptive magnetic configurations in strongly sheared active
regions and filament channels carry substantial net currents. We discuss
the implications of this result for the modeling of solar eruptions.
Title: Thermal energy creation and transport and X-ray/EUV emission
in a thermodynamic MHD CME simulation
Authors: Reeves, Kathy; Mikić, Zoran; Linker, Jon; Török, Tibor
Bibcode: 2014shin.confE...2R
Altcode:
We model a CME using a 3D numerical MHD code that includes coronal
heating, thermal conduction and radiative cooling in the energy
equation. We first develop a global coronal solution (from 1 to 20 Rs)
to serve as the initial condition for the CME simulation. The magnetic
flux distribution at 1 Rs is produced by a localized subsurface dipole
superimposed on a global dipole field, to mimic the presence of an
active region within the global corona. The resulting configuration
has solar wind emanating from the open field regions, dense plasma in
the streamer belt, and hot plasma in the active region. We introduce
transverse electric fields near the neutral line in the active region
to form a flux rope, then a converging flow is imposed that causes the
eruption. We follow the quantities responsible for plasma heating
and cooling during the eruption, including thermal conduction,
radiation, adiabatic compression and expansion, coronal heating
and ohmic heating due to dissipation of currents. We find that the
adiabatic compression plays an important role in heating plasma around
the current sheet and in the collapsing reconnected loops under the
erupting flux rope. Thermal conduction also plays an important role in
the transport of thermal energy. We follow the formation and evolution
of the current sheet and simulate emissions in the X-ray and extreme
ultra-violet wavelengths in order to determine signatures of current
sheet energetics in observations from the XRT on the Hinode satellite
and the AIA instrument on the Solar Dynamics Observatory.
Title: A Method for Embedding Circular Force-Free Flux Ropes in
Potential Magnetic Fields
Authors: Titov, Viacheslav; Torok, Tibor; Mikic, Zoran; Linker, Jon A.
Bibcode: 2014AAS...22421204T
Altcode:
We propose a method for constructing approximate force-free equilibria
in pre-eruptive configurations that locally are a bipolar-type
potential magnetic field with a thin force-free flux rope embedded
inside it. The flux rope is assumed to have a circular-arc axis,
circular cross-section, and electric current that is either concentrated
in a thin layer at the boundary of the rope or smoothly distributed
across it with a maximum of the current density at the center.The
entire solution is described in terms of the magnetic vector
potential in order to facilitate the implementation of the method
in numerical magnetohydrodynamic (MHD) codes that evolve the vector
potential rather than the magnetic field itself. The parameters of
the flux rope can be chosen so that its subsequent MHD relaxation
under photospheric line-tied boundary conditions leads to nearly
exact numerical equilibria. To show the capabilities of our method,
we apply it to several cases with different ambient magnetic fields
and internal flux-rope structures. These examples demonstrate that
the proposed method is a useful tool for initializing data-driven
simulations of solar eruptions.
Title: Comparative Study of MHD Modeling of the Background Solar Wind
Authors: Gressl, C.; Veronig, A. M.; Temmer, M.; Odstrčil, D.;
Linker, J. A.; Mikić, Z.; Riley, P.
Bibcode: 2014SoPh..289.1783G
Altcode: 2013arXiv1312.1220G
Knowledge about the background solar wind plays a crucial role in
the framework of space-weather forecasting. In-situ measurements
of the background solar wind are only available for a few points in
the heliosphere where spacecraft are located, therefore we have to
rely on heliospheric models to derive the distribution of solar-wind
parameters in interplanetary space. We test the performance of different
solar-wind models, namely Magnetohydrodynamic Algorithm outside
a Sphere/ENLIL (MAS/ENLIL), Wang-Sheeley-Arge/ENLIL (WSA/ENLIL),
and MAS/MAS, by comparing model results with in-situ measurements
from spacecraft located at 1 AU distance to the Sun (ACE, Wind). To
exclude the influence of interplanetary coronal mass ejections
(ICMEs), we chose the year 2007 as a time period with low solar
activity for our comparison. We found that the general structure of the
background solar wind is well reproduced by all models. The best model
results were obtained for the parameter solar-wind speed. However,
the predicted arrival times of high-speed solar-wind streams have
typical uncertainties of the order of about one day. Comparison of
model runs with synoptic magnetic maps from different observatories
revealed that the choice of the synoptic map significantly affects
the model performance.
Title: Validating a Time-dependent Turbulence-driven Model of the
Solar Wind
Authors: Lionello, Roberto; Velli, Marco; Downs, Cooper; Linker,
Jon A.; Mikić, Zoran; Verdini, Andrea
Bibcode: 2014ApJ...784..120L
Altcode: 2014arXiv1402.4188L
Although the mechanisms responsible for heating the Sun's corona and
accelerating the solar wind are still being actively investigated, it
is largely accepted that photospheric motions provide the energy source
and that the magnetic field must play a key role in the process. Verdini
et al. presented a model for heating and accelerating the solar wind
based on the turbulent dissipation of Alfvén waves. We first use a
time-dependent model of the solar wind to reproduce one of Verdini et
al.'s solutions; then, we extend its application to the case where the
energy equation includes thermal conduction and radiation losses, and
the upper chromosphere is part of the computational domain. Using this
model, we explore the parameter space and describe the characteristics
of a fast solar wind solution. We discuss how this formulation may be
applied to a three-dimensional MHD model of the corona and solar wind.
Title: A Multi-Observatory Inter-Comparison of Line-of-Sight Synoptic
Solar Magnetograms
Authors: Riley, P.; Ben-Nun, M.; Linker, J. A.; Mikic, Z.; Svalgaard,
L.; Harvey, J.; Bertello, L.; Hoeksema, T.; Liu, Y.; Ulrich, R.
Bibcode: 2014SoPh..289..769R
Altcode:
The observed photospheric magnetic field is a crucial parameter
for understanding a range of fundamental solar and heliospheric
phenomena. Synoptic maps, in particular, which are derived from
the observed line-of-sight photospheric magnetic field and built up
over a period of 27 days, are the main driver for global numerical
models of the solar corona and inner heliosphere. Yet, in spite of 60
years of measurements, quantitative estimates remain elusive. In this
study, we compare maps from seven solar observatories (Stanford/WSO,
NSO/KPVT, NSO/SOLIS, NSO/GONG, SOHO/MDI, UCLA/MWO, and SDO /HMI)
to identify consistencies and differences among them. We find that
while there is a general qualitative consensus, there are also some
significant differences. We compute conversion factors that relate
measurements made by one observatory to another using both synoptic
map pixel-by-pixel and histogram-equating techniques, and we also
estimate the correlation between datasets. For example, Wilcox Solar
Observatory (WSO) synoptic maps must be multiplied by a factor of 3 -
4 to match Mount Wilson Observatory (MWO) estimates. Additionally, we
find no evidence that the MWO saturation correction factor should be
applied to WSO data, as has been done in previous studies. Finally,
we explore the relationship between these datasets over more than
a solar cycle, demonstrating that, with a few notable exceptions,
the conversion factors remain relatively constant. While our study
was able to quantitatively describe the relationship between the
datasets, it did not uncover any obvious "ground truth." We offer
several suggestions for how this may be addressed in the future.
Title: Distribution of Electric Currents in Solar Active Regions
Authors: Török, T.; Leake, J. E.; Titov, V. S.; Archontis, V.;
Mikić, Z.; Linton, M. G.; Dalmasse, K.; Aulanier, G.; Kliem, B.
Bibcode: 2014ApJ...782L..10T
Altcode: 2014arXiv1401.2931T
There has been a long-standing debate on the question of whether or
not electric currents in solar active regions are neutralized. That
is, whether or not the main (or direct) coronal currents connecting
the active region polarities are surrounded by shielding (or return)
currents of equal total value and opposite direction. Both theory and
observations are not yet fully conclusive regarding this question, and
numerical simulations have, surprisingly, barely been used to address
it. Here we quantify the evolution of electric currents during the
formation of a bipolar active region by considering a three-dimensional
magnetohydrodynamic simulation of the emergence of a sub-photospheric,
current-neutralized magnetic flux rope into the solar atmosphere. We
find that a strong deviation from current neutralization develops
simultaneously with the onset of significant flux emergence into the
corona, accompanied by the development of substantial magnetic shear
along the active region's polarity inversion line. After the region
has formed and flux emergence has ceased, the strong magnetic fields
in the region's center are connected solely by direct currents, and
the total direct current is several times larger than the total return
current. These results suggest that active regions, the main sources
of coronal mass ejections and flares, are born with substantial net
currents, in agreement with recent observations. Furthermore, they
support eruption models that employ pre-eruption magnetic fields
containing such currents.
Title: Global Magnetic Topology and Large-Scale Dynamics of the
Solar Corona
Authors: Titov, Viacheslav; Linker, Jon; Mikic, Zoran; Riley, Pete;
Lionello, Roberto; Downs, Cooper; Torok, Tibor
Bibcode: 2014cosp...40E3350T
Altcode:
We consider the global topology of the coronal magnetic field
in relation to the large-scale dynamics of the solar corona. Our
consideration includes recent results on the structural analysis
of this field determined in two different approximations, namely,
potential field source surface model and solar magnetohydrodynamic
model. We identify similarities and differences between structural
features of the magnetic field obtained in these two models and discuss
their implications for understanding various large-scale phenomena in
the solar corona. The underlying magnetic topology manifests itself
in a variety of observed morphological features such as streamers,
pseudo-streamers or unipolar streamers, EUV dimmings, flare ribbons,
coronal holes, and jets. For each of them, the related magnetic
configuration has specific structural features, whose presence has to be
not only identified but also verified on its independence from the used
field model in order to reliably predict the impact of such features on
physical processes in the corona. Among them are magnetic null points
and minima, bald patches, separatrix surfaces and quasi-separatrix
layers, and open and closed separator field lines. These features form
a structural skeleton of the coronal magnetic field and are directly
involved through the ubiquitous process of magnetic reconnection in many
solar dynamic phenomena such as coronal mass ejections, solar wind,
acceleration and transport of energetic particles. We will pinpoint
and elucidate in our overview some of such involvements that have
recently received a considerable attention in our ongoing projects at
Predictive Science.
Title: Time-Dependent Coupled Coronal-Solar Wind-SEP Modeling
Authors: Linker, Jon; Mikic, Zoran; Schwadron, Nathan; Riley, Pete;
Gorby, Matthew; Lionello, Roberto; Downs, Cooper; Torok, Tibor
Bibcode: 2014cosp...40E1840L
Altcode:
Solar energetic particle (SEP) events are important space weather
phenomena. SEPs can damage satellite instrumentation, and they can be
hazardous for crews of Low Earth Orbit spacecraft and the International
Space Station, especially when engaged in extravehicular activity. The
acceleration and transport of SEPs is intimately tied to the evolution
and propagation of coronal mass ejections (CMEs) and their associated
shock waves. In this presentation, we describe an approach to modeling
CMEs in the corona and inner heliosphere, together with modeling of
SEP acceleration and transport. CMEs are initiated and followed in
a realistic corona and solar wind using the MAS MHD code, and SEPs
are modeled using EPREM, a 3D energetic particle transport code. The
particles are not truly coupled to the MHD solution, in the sense
that the electric and magnetic fields from the MHD computation drive
the solutions of the focused transport equation. We show initial
comparisons with typical CME observations and SEP data, and discuss
the strengths and limitations of this approach.
Title: Data-Driven Global Coronal Models
Authors: Linker, Jon; Mikic, Zoran; Riley, Pete; Henney, Carl; Arge,
Charles; Lionello, Roberto; Downs, Cooper
Bibcode: 2014cosp...40E1839L
Altcode:
MHD simulations of the solar corona based on maps of the solar
magnetic field have been demonstrated to describe many aspects of
coronal structure. However, these models are typically integrated to
steady state, using synoptic or daily-updated magnetic maps to derive
the boundary conditions. The Sun's magnetic flux is always evolving,
and these changes in the flux affect the structure and dynamics of the
corona and heliosphere. In this presentation, we describe an approach to
evolutionary models of the corona and solar wind, using time-dependent
boundary conditions. A key aspect of our approach is the use of the Air
Force Data Assimilative Photospheric flux Transport (ADAPT) model to
develop time-evolving boundary conditions for the magnetic field. ADAPT
incorporates data assimilation techniques into the Worden and Harvey
(2000) flux evolution model, making it an especially suitable candidate
for providing boundary conditions to MHD models. We describe initial
results and their implications for coronal hole evolution and the
origin of the slow solar wind.
Title: Magnetohydrodynamic Simulations of Interplanetary Coronal
Mass Ejections
Authors: Lionello, Roberto; Downs, Cooper; Linker, Jon A.; Török,
Tibor; Riley, Pete; Mikić, Zoran
Bibcode: 2013ApJ...777...76L
Altcode:
We describe a new MHD model for the propagation of interplanetary
coronal mass ejections (ICMEs) in the solar wind. Accurately following
the propagation of ICMEs is important for determining space weather
conditions. Our model solves the MHD equations in spherical coordinates
from a lower boundary above the critical point to Earth and beyond. On
this spherical surface, we prescribe the magnetic field, velocity,
density, and temperature calculated typically directly from a coronal
MHD model as time-dependent boundary conditions. However, any model
that can provide such quantities either in the inertial or rotating
frame of the Sun is suitable. We present two validations of the
technique employed in our new model and a more realistic simulation
of the propagation of an ICME from the Sun to Earth.
Title: Thermal Non-equilibrium Revisited: A Heating Model for
Coronal Loops
Authors: Lionello, Roberto; Winebarger, Amy R.; Mok, Yung; Linker,
Jon A.; Mikić, Zoran
Bibcode: 2013ApJ...773..134L
Altcode:
The location and frequency of events that heat the million-degree corona
are still a matter of debate. One potential heating scenario is that
the energy release is effectively steady and highly localized at the
footpoints of coronal structures. Such an energy deposition drives
thermal non-equilibrium solutions in the hydrodynamic equations in
longer loops. This heating scenario was considered and discarded by
Klimchuk et al. on the basis of their one-dimensional simulations as
incapable of reproducing observational characteristics of loops. In
this paper, we use three-dimensional simulations to generate synthetic
emission images, from which we select and analyze six loops. The main
differences between our model and that of Klimchuk et al. concern (1)
dimensionality, (2) resolution, (3) geometrical properties of the loops,
(4) heating function, and (5) radiative function. We find evidence,
in this small set of simulated loops, that the evolution of the light
curves, the variation of temperature along the loops, the density
profile, and the absence of small-scale structures are compatible with
the characteristics of observed loops. We conclude that quasi-steady
footpoint heating that drives thermal non-equilibrium solutions cannot
yet be ruled out as a viable heating scenario for EUV loops.
Title: Non-equilibrium Ionization Modeling of the Current Sheet in
a Simulated Solar Eruption
Authors: Shen, Chengcai; Reeves, Katharine K.; Raymond, John C.;
Murphy, Nicholas A.; Ko, Yuan-Kuen; Lin, Jun; Mikić, Zoran; Linker,
Jon A.
Bibcode: 2013ApJ...773..110S
Altcode:
The current sheet that extends from the top of flare loops and connects
to an associated flux rope is a common structure in models of coronal
mass ejections (CMEs). To understand the observational properties of
CME current sheets, we generated predictions from a flare/CME model
to be compared with observations. We use a simulation of a large-scale
CME current sheet previously reported by Reeves et al. This simulation
includes ohmic and coronal heating, thermal conduction, and radiative
cooling in the energy equation. Using the results of this simulation,
we perform time-dependent ionization calculations of the flow in a
CME current sheet and construct two-dimensional spatial distributions
of ionic charge states for multiple chemical elements. We use the
filter responses from the Atmospheric Imaging Assembly (AIA) on the
Solar Dynamics Observatory and the predicted intensities of emission
lines to compute the count rates for each of the AIA bands. The
results show differences in the emission line intensities between
equilibrium and non-equilibrium ionization. The current sheet plasma
is underionized at low heights and overionized at large heights. At
low heights in the current sheet, the intensities of the AIA 94 Å
and 131 Å channels are lower for non-equilibrium ionization than for
equilibrium ionization. At large heights, these intensities are higher
for non-equilibrium ionization than for equilibrium ionization inside
the current sheet. The assumption of ionization equilibrium would
lead to a significant underestimate of the temperature low in the
current sheet and overestimate at larger heights. We also calculate
the intensities of ultraviolet lines and predict emission features to
be compared with events from the Ultraviolet Coronagraph Spectrometer
on the Solar and Heliospheric Observatory, including a low-intensity
region around the current sheet corresponding to this model.
Title: The Importance of Geometric Effects in Coronal Loop Models
Authors: Mikić, Zoran; Lionello, Roberto; Mok, Yung; Linker, Jon A.;
Winebarger, Amy R.
Bibcode: 2013ApJ...773...94M
Altcode:
We systematically investigate the effects of geometrical assumptions in
one-dimensional (1D) models of coronal loops. Many investigations of
coronal loops have been based on restrictive assumptions, including
symmetry in the loop shape and heating profile, and a uniform
cross-sectional area. Starting with a solution for a symmetric
uniform-area loop with uniform heating, we gradually relax these
restrictive assumptions to consider the effects of nonuniform area,
nonuniform heating, a nonsymmetric loop shape, and nonsymmetric
heating, to show that the character of the solutions can change in
important ways. We find that loops with nonuniform cross-sectional
area are more likely to experience thermal nonequilibrium, and that
they produce significantly enhanced coronal emission, compared with
their uniform-area counterparts. We identify a process of incomplete
condensation in loops experiencing thermal nonequilibrium during
which the coronal parts of loops never fully cool to chromospheric
temperatures. These solutions are characterized by persistent siphon
flows. Their properties agree with observations (Lionello et al.) and
may not suffer from the drawbacks that led Klimchuk et al. to conclude
that thermal nonequilibrium is not consistent with observations. We
show that our 1D results are qualitatively similar to those seen in a
three-dimensional model of an active region. Our results suggest that
thermal nonequilibrium may play an important role in the behavior
of coronal loops, and that its dismissal by Klimchuk et al., whose
model suffered from some of the restrictive assumptions we described,
may have been premature.
Title: Probing the Solar Magnetic Field With a Sun-Grazing Comet
Authors: Downs, Cooper; Linker, J. A.; Mikic, Z.; Riley, P.; Schrijver,
C. J.; Saint-Hilaire, P.
Bibcode: 2013SPD....4430503D
Altcode:
Observations of comets occupy a rich history within Solar and
Heliospheric science. Cometary plasma tails probe the solar wind in
the inner solar system 0.5-3 AU) and their observations led to its
discovery more than half a century ago. Fast forwarding to today,
recent observations of sun-grazing comets within the solar corona
have opened up a whole new avenue to study the Sun with these striking
celestial bodies. Here we present our recent study of the perihelion
passage of comet C/2011 W3 (Lovejoy), which came within 140Mm of
the solar surface. Imaged from multiple perspectives by SDO/AIA and
the STEREO/EUVI, extreme ultraviolet (EUV) observations of Lovejoy's
tail showed substantial changes in direction, intensity, magnitude,
and persistence. To understand this unique signature, we combine a
state-of-the-art magnetohydrodynamic (MHD) model of the solar corona
and a prescription for the motion of emitting cometary tail ions in
an embedded plasma. We show how the observed tail motions reveal the
inhomogeneous magnetic field of the solar corona, and demonstrate
how they constrain field and plasma properties in a region where the
coronal plasma is normally not easily observed in EUV. We will also
discuss our results in context of the upcoming perihelion passage of
comet C/2012 S1 (ISON), expected by many to be a spectacular probe of
the near-sun environment. Work supported by NASA and NSF.
Title: Thermal Structure of Coronal Loops
Authors: Mok, Yung; Lionello, R.; Mikic, Z.; Linker, J. A.
Bibcode: 2013SPD....44...26M
Altcode:
We investigated the internal structure of a coronal loop that occurs
naturally in our 3D simulation based on an Alfven turbulence heating
model. The thermal structure above the active region is in a thermal
non-equilibrium state. As the system evolves, it develops a tunnel-like
low-temperature region along a small bundle of field lines. The EUV
emissivity of this structure also exhibits a coronal loop along this
temperature tunnel, which has a nearly uniform cross section. Over the
course of about 27 hours solar time, multiple loops occur at discrete
locations. Some of them carry high-speed plasma flows. The structure
and dynamics of these loops will be analyzed.
Title: A Time-Dependent Turbulence-Driven Model of the Solar Wind
Authors: Lionello, Roberto; Downs, C.; Linker, J. A.; Mikic, Z.;
Velli, M.; Verdini, A.
Bibcode: 2013SPD....44...22L
Altcode:
Although the mechanisms responsible for heating the Sun's corona and
accelerating the solar wind are still actively investigated, it is
largely accepted that photospheric motions provide the energy source
and that the magnetic field must play a key role in the process. Verdini
et al. (2010) presented a model for heating and accelerating the solar
wind based on the turbulent dissipation of Alfven waves. We first
use our time-dependent model of the solar wind to reproduce Verdini
et al's solution; then we extend its application to the case when the
energy equation includes thermal conduction and radiation losses, and
the upper chromosphere is part of the computational domain.Application
of this formulation to our 3D MHD model of the solar corona and solar
wind will be discussed.
Title: Coronal Modeling and Synchronic Maps
Authors: Linker, Jon A.; Lionello, R.; Mikic, Z.; Riley, P.; Downs,
C.; Henney, C. J.; Arge, C.
Bibcode: 2013SPD....4430504L
Altcode:
MHD simulations of the solar corona rely on maps of the solar
magnetic field (typically measured at the photosphere) for input as
boundary conditions. These "synoptic" maps (available from a number
of ground-based and space-based solar observatories), which are
perhaps better described as "diachronic," are built up over a solar
rotation. A well-known problem with this approach is that the maps
contain data that is as much as 27 days old. The Sun's magnetic flux
is always evolving, and these changes in the flux affect coronal and
heliospheric structure. Flux evolution models can in principle provide
a more accurate specification, by estimating the likely state of the
photospheric magnetic field on unobserved portions of the Sun. The
Air Force Data Assimilative Photospheric flux Transport (ADAPT) model
(Arge et al. 2010), which incorporates data assimilation techniques
into the Worden and Harvey (2000) flux evolution model, is especially
well-suited for this purpose. In this presentation we describe the
use of such "synchronic" maps with coronal models. We compare results
using synchronic maps versus the traditional synoptic maps. Research
supported by AFOSR, NASA, and NSF.
Title: Structure of the Coronal Streamers During Solar Minimum
and Maximum
Authors: Kramar, Maxim; Davila, J.; Mikic, Z.
Bibcode: 2013SPD....44...11K
Altcode:
We analyze the meridional cross-section of the 3D coronal electron
density in the range from 1.5 to 4 R_⊙ obtained by the tomography
method during minimum and beginning of maximum of solar activity
corresponding to February 2008 and July 2011, respectively. The
importance of this coronal region is that it contains the transition
from closed to open magnetic coronal structures. At the moment,
only STEREO/COR1 provides observation that provides information
on the coronal structure in this region. Therefore, analysis of 3D
coronal density structure is critical for deriving the position where
transition from closed to open magnetic coronal structures occurs. The
3D coronal density in the region of interest has been obtained
by applying tomographic techniques to white light coronagraph data
obtained by STEREO/COR1 instrument. It is shown that enhanced density
structures associated with coronal streamers and pseudostreamers have
a tendency to become radially directed at heliocentric distances of
about 3 and 2 $R_\odot$ during minimum and maximum of solar activity,
respectively. Potential Field models (PFSS) with several different
values of the Source Surface position have been analyzed for consistency
with the obtained 3D coronal density structure.
Title: The challenge in making models of fast CMEs
Authors: Mikić, Zoran; Török, Tibor; Titov, Viacheslav; Linker,
Jon A.; Lionello, Roberto; Downs, Cooper; Riley, Pete
Bibcode: 2013AIPC.1539...42M
Altcode:
It has been a challenge to explain theoretically how fast CMEs
(exceeding ~ 1,000km/s) occur. Our numerical models suggest that it
is not easy to release enough magnetic energy impulsively from an
active region. We have been studying CME models that are constrained
by observed magnetic fields, with realistic coronal plasma density
and temperature profiles, as derived from thermodynamic models of
the corona. We find that to get fast CMEs, the important parameters
are the magnetic energy density, the magnetic field drop-off index,
and the Alfvén speed profile in active regions. We describe how we
energize active regions, and how we subsequently initiate CMEs via
flux cancellation. We contrast CMEs from idealized zero-beta models
with more sophisticated models based on thermodynamic solutions.
Title: Coronal and heliospheric modeling using flux-evolved maps
Authors: Linker, Jon A.; Mikić, Zoran; Riley, Pete; Downs, Cooper;
Lionello, Roberto; Henney, Carl; Arge, Charles N.
Bibcode: 2013AIPC.1539...26L
Altcode:
Magnetohydrodynamic (MHD) simulations are now routinely used
to produce models of the solar corona and inner heliosphere for
specific time periods. These models typically rely on maps of the
photospheric magnetic field. Two well-known problems arise from
the use of these "synoptic" maps. First, the Sun's poles are poorly
observed, which necessarily means that the polar fields in these maps
must be reconstructed with a variety of interpolation/extrapolation
techniques. Second, the synoptic maps contain data that is as much as
27 days old, whereas the Sun's magnetic flux is always evolving. Flux
evolution models can in principle alleviate both these difficulties,
by providing physical approximations for the polar fields and by
estimating the likely state of the field on unobserved portions of
the Sun. In this study, we focus on the polar field problem, and show
why typical synoptic maps may underestimate the polar magnetic fields
near solar minimum. We use a map created with the Air Force Data
Assimilative Photospheric flux Transport (ADAPT)model to show that
it contains significantly higher polar fluxes than a corresponding
synoptic map with polar fields filled by extrapolation. We then
show with potential field source-surface (PFSS) models that this
map produces higher estimates for the interplanetary magnetic field,
which correspond more closely to observations.
Title: Which magnetic topologies are favorable for an efficient
acceleration and escape of SEPs?
Authors: Titov, Viacheslav S.; Linker, Jon A.; Mikić, Zoran; Török,
Tibor; Lionello, Roberto
Bibcode: 2013shin.confE.129T
Altcode:
We assume that unstable magnetic flux ropes are the drivers of solar
flares and CMEs producing SEPs. The natural sites for the acceleration
of SEPs are current sheets and shocks that are formed in the solar
corona around these flux ropes during their eruptions. The location of
the current sheets and shocks in turn depends on the structure of the
background magnetic field ambient to the erupting flux ropes. This
raises an important question on which topologies of the background
field are favorable for an efficient production and escape of SEPs. We
propose that such topologies are inherent to pseudo-streamers, whose
lobes often harbor magnetic flux ropes. The pseudo-streamers possess
closed and open separator field lines, where current sheets have to
be formed whenever the harbored flux ropes start to erupt. These are
good preconditions for both the acceleration and transport of SEPs in
the open-field corona. In addition, the pseudo-streamers' structure
is prone to the generation of sympathetic flux-rope eruptions, which
can produce widely separated but well-synchronized beams of SEPs.
Title: Coronal models
Authors: Linker, Jon A.; Lionello, Roberto; Downs, Cooper; Riley,
Pete; Mikic, Zoran
Bibcode: 2013shin.confE..20L
Altcode:
Coronal holes are regions that appear dark in EUV and X-ray emission
on the solar disk. The general qualitative picture of these regions,
that they are magnetically open and are the origin of the fast solar
wind, has strong observational support. However, many details of this
picture are unclear, such as: What happens at the edges of coronal
holes? Are the edges primarily steady structures, or are they opening
and closing? Is the open magnetic flux at 1 AU well-accounted for by
the magnetic flux emanating from coronal holes? What do disagreements
in coronal hole boundaries from different emission lines tell us about
there structure? In this talk we will discuss how these questions can
be approached from a theoretical and modeling perspective.
Title: Integrating physics-based coronal heating and solar wind
acceleration in a global MHD model
Authors: Lionello, Roberto; Velli, Marco; Linker, Jon A.; Mikić, Zoran
Bibcode: 2013AIPC.1539...30L
Altcode:
Although the mechanisms responsible for heating the Sun's corona
and accelerating the solar wind are being actively investigated,
it is largely accepted that photospheric motions provide the energy
source and that the magnetic field must play a key role in the
process. 3D MHD models of the corona and of the solar wind usually
employ some phenomenological artifice to accelerate the wind and heat
the corona. Within the framework of a 3D full MHD numerical code,
we apply the results of Rappazzo et al. [1] on turbulent cascade to
heat the closed-field region, and those of Verdini and Velli [2],
Verdini et al. [3] on Alfvén turbulence dissipation to accelerate
the solar wind. We also compare the properties of the solar wind at
1 A.U. using a formulation based on Verdini and Velli [2], Verdini et
al. [3] and one based on Chandran et al. [4].
Title: Empirical Coronal Heating Models Inspired by Wave Turbulence
Authors: Downs, Cooper; Lionello, Roberto; Linker, Jon A.; Mikic, Zoran
Bibcode: 2013shin.confE..83D
Altcode:
To simulate the energy balance of coronal plasmas on macroscopic scales,
we often require the specification of the coronal heating mechanism
in some functional form. Traditionally our group has used empirical
methods; tailoring analytic functions, such as exponentials, to
produce a 3D, multi-thermal corona that is consistent with EUV imaging
observations. To build a more physically motivated heating function,
we investigate a wave-turbulence dissipation (WTD) phenomenology for
the heating of closed coronal loops. We discuss an implementation of
non-WKB equations for the propagation and dissipation of wave turbulence
designed to capture the basic features of coronal heating. The parameter
space of this model is explored for an idealized loop, and for one
hundred active region and quiet sun loops extracted from a data-driven
MHD solution. We find that by solving auxiliary WTD equations for the
coronal heating function along the loop, we can significantly reduce
the number of empirical free parameters and still adequately describe
a plethora of coronal conditions within a single heating model. This is
due to the implicit dependence of the WTD equations on loop properties
(both along the loop and at the footpoints).
Title: Pseudo-Streamer Structures in the 2010 August 1-2 CMEs:
PFSS verses MHD model.
Authors: Titov, Viacheslav S.; Mikić, Zoran; Török, Tibor; Linker,
Jon A.; Panasenco, Olga
Bibcode: 2013shin.confE.130T
Altcode:
We upgrade our previous potential field source-surface (PFSS) model of
the background magnetic field in the 2010 August 1-2 sympathetic CMEs
to a more accurate thermodynamic MHD model of the solar corona. For
this new model, we verify our earlier results on the structure of the
large-scale magnetic field, making a similar topological analysis of the
field as before. We identify the similarities and differences between
the two configurations, particularly, for the eruptive regions with
three pseudo-streamers that we have found before. The new study confirms
that all these pseudo-streamers indeed contain vertical separatrix
surfaces located between two adjacent disconnected coronal holes. Of
special interest to us are the magnetic null points and separator
field lines belonging to such separatrix surfaces. These topological
features exist in both PFSS and MHD models, albeit in different
forms. We reassert our earlier hypothesis that magnetic reconnection
at these nulls and separators likely plays a key role in establishing
a physical connection between the successive eruptions observed by
SDO and STEREO. The results obtained provide further validation of
our recent simplified MHD model of sympathetic eruptions (Török et
al. 2011). Work supported by Lockheed Martin, NASA's Heliophysics
Theory and SR&T programs, and SHINE NSF Grant AGS-1156119.
Title: Ensemble modeling of the ambient solar wind
Authors: Riley, Pete; Linker, Jon A.; Mikič, Zoran
Bibcode: 2013AIPC.1539..259R
Altcode:
Ensemble modeling is a method of prediction based on the use of a
representative sample of possible future states. Global models of the
solar corona and inner heliosphere are now maturing to the point of
becoming predictive tools, thus, it is both meaningful and necessary
to quantitatively assess their uncertainty and limitations. In this
study, we apply simple ensemble modeling techniques in a first step
towards these goals. We focus on one relatively quiescent time period,
Carrington rotation 2062, which occurred during the late declining phase
of solar cycle 23 and assess the sensitivity of the model results to
variations in boundary conditions, models, and free parameter values. We
present variance maps, "whisker" plots, and Taylor diagrams to estimate
the accuracy of the solutions, which demonstrate that the ensemble
mean solution outperforms any of the individual realizations. Our
results provide a baseline against which future model improvements
can be compared.
Title: Probing the Solar Magnetic Field with a Sun-Grazing Comet
Authors: Downs, Cooper; Linker, Jon A.; Mikić, Zoran; Riley, Pete;
Schrijver, Carolus J.; Saint-Hilaire, Pascal
Bibcode: 2013Sci...340.1196D
Altcode:
On 15 and 16 December 2011, Sun-grazing comet C/2011 W3 (Lovejoy)
passed deep within the solar corona, effectively probing a region
that has never been visited by spacecraft. Imaged from multiple
perspectives, extreme ultraviolet observations of Lovejoy's tail
showed substantial changes in direction, intensity, magnitude,
and persistence. To understand this unique signature, we combined a
state-of-the-art magnetohydrodynamic model of the solar corona and
a model for the motion of emitting cometary tail ions in an embedded
plasma. The observed tail motions reveal the inhomogeneous magnetic
field of the solar corona. We show how these motions constrain field
and plasma properties along the trajectory, and how they can be used to
meaningfully distinguish between two classes of magnetic field models.
Title: Reconstruction of the solar coronal magnetic field in
spherical geometry
Authors: Amari, T.; Aly, J. -J.; Canou, A.; Mikic, Z.
Bibcode: 2013A&A...553A..43A
Altcode:
Context. High-resolution vector magnetographs either onboard spacecrafts
or satellites (HMI/SDO, etc.) or ground based (SOLIS, etc.) now
gives access to vector synoptic maps, composite magnetograms made of
multiple interactive active regions, and full disk magnetograms. It
thus become possible to reconstruct the coronal magnetic field on the
full Sun scale.
Aims: We present a method for reconstructing
the global solar coronal magnetic field as a nonlinear force-free
field. It is based on a well-posed Grad-Rubin iterative scheme adapted
to spherical coordinates
Methods: This method is a natural
extension to spherical geometry of the one we previously developed in
Cartesian geometry. It is implemented in the code XTRAPOLS, which
is a massively parallel code. It allows dealing with the strong
constraints put on the computational methods by having to handle the
very large amounts of data contained in high-resolution large-scale
magnetograms. The method exploits the mixed elliptic-hyperbolic nature
of the Grad-Rubin boundary value problem. It uses a finite-difference
method for the elliptic part and a method of characteristics for the
hyperbolic part. The computed field guarantees to be divergence free
up to round-off errors, by introducing a representation in terms of a
vector potential satisfying specific gauge conditions. The construction
of the latter - called here the restricted DeVore gauge - is described
in detail in an appendix.
Results: We show that XTRAPOLS performs
well by applying it to the reconstruction of a particular semi-analytic
force-free field that has already been considered by various authors.
Title: Numerical modeling of fast CMEs from Sun to Earth
Authors: Torok, Tibor; Downs, Cooper; Lionello, Roberto; Linker,
Jon A.; Titov, Viacheslav S.; Mikic, Zoran; Riley, Pete
Bibcode: 2013EGUGA..1512485T
Altcode:
Coronal mass ejections (CMEs) are the main driver of space weather
disturbances near Earth. The most severe disturbances are caused
by fast CMEs with coronal speeds in excess of 1000 km/s and magnetic
orientations favorable for interaction with the Earth's magnetosphere. A
proper assessment of the impact of CMEs from numerical simulations
requires the self-consistent modeling of both CME initiation and
its propagation through interplanetary space. Such simulations are
very challenging, in particular because of the enormous disparity of
scales involved. Here we present our recent attempts to model fast
CMEs all the way from Sun to Earth. We first simulate the initiation
and propagation of CMEs in the corona using our "thermodynamic" MHD
model, which includes empirical coronal heating, thermal conduction,
and radiation losses. After the initial configuration, consisting of a
large-scale dipole field and an idealized active region, is relaxed to
a steady-state solar wind solution, we insert a flux rope in magnetic
equilibrium into the active region and trigger its eruption by imposing
localized converging flows. We perform a small series of simulations,
varying the geometry and field strength of the flux rope. The resulting
CMEs produce a shock low in the corona and reach peak velocities of
up to 3000 km/s, after which they slow down to constant propagation
speeds of 1000 km/s or less. We then use our recently developed
heliospheric model to simulate the further propagation to 1 AU for
one of the model CMEs.
Title: Non-Equilibrium Ionization Modeling of the Current Sheet in
a Simulated Solar Eruption
Authors: Shen, C.; Reeves, K. K.; Raymond, J. C.; Murphy, N. A.; Ko,
Y. -K.; Lin, J.; Mikić, Z.
Bibcode: 2013enss.confE..44S
Altcode:
The current sheet that extends from the top of flare loops to an
associated flux rope is a common structure in models of coronal mass
ejections (CMEs). To understand the observational properties of CME
current sheets, we generate predictions from flare/CME models to
be compared with observations. We use a simulation of a large-scale
CME current sheet previously reported by Reeves et al. (2010). This
simulation includes Ohmic and coronal heating, thermal conduction,
and radiative cooling in the energy equation. Using the results of
this simulation, we perform time-dependent ionization calculations of
the flow in a CME current sheet and construct two-dimensional spatial
distributions of ionic charge states for multiple chemical elements. We
use the filter responses from the Atmospheric Imaging Assembly (AIA) on
the Solar Dynamics Observatory and the predicted intensities of emission
lines to compute the count rates for each of the AIA bands. The results
show differences in the emission line intensities between equilibrium
and non-equilibrium ionization. The current sheet plasma is underionized
at low heights and overionized at large heights. At low heights in the
current sheet, the intensities of the AIA 94Å and 131Å channels are
lower for non-equilibrium ionization than for equilibrium ionization;
and at large heights, these intensities are higher for non-equilibrium
ionization than for equilibrium ionization. We also calculated the
intensity of ultraviolet lines and predicted emission features that
could be compared with those events observed by the Ultraviolet
Coronagraph Spectrometer on the Solar and Heliospheric Observatory,
including a low intensity region around the sheet present in the model.
Title: Can We Predict the Geoeffectiveness of CMEs?
Authors: Linker, Jon; Lionello, Roberto; Downs, Cooper; Mikic, Zoran;
Torok, Tibor; Titov, Viacheslav; Riley, Pete
Bibcode: 2013enss.confE..11L
Altcode:
Coronal Mass Ejections (CMEs) are immense eruptions of plasma and
magnetic field that are propelled outward from the sun, sometimes
with velocities greater than 2000 km/s. They are also responsible for
some of the most severe space weather at Earth, including geomagnetic
storms. Modeling CMEs from Sun to Earth is especially challenging,
because of the enormous disparity of scales involved. At the present
time, both NOAA SWPC and the CCMC use the WSA-Enlil model with "cone
model" CMEs to predict the arrival of possibly geoeffective CMEs at
Earth. This model has no embedded magnetic fields in the CME, and
therefore does not successfully predict the magnitude and direction
of Bz. In this paper, we outline a possible approach to this problem,
using coupled coronal and heliospheric simulations of coronal mass
ejections. Research supported by NASA, NSF, and AFOSR.
Title: Characterizing the Magnetic Topology of Solar Eruptions
Authors: Titov, Viacheslav S.; Mikic, Zoran; Torok, Tibor; Linker,
Jon A.; Lionello, Roberto; Riley, Pete
Bibcode: 2013enss.confE..15T
Altcode:
Numerical MHD simulations of solar eruptions have made it possible
to model the evolution of magnetic configurations with considerable
realism. However, a comprehensive understanding of these complex
configurations requires the development of sophisticated techniques to
analyze the three-dimensional magnetic field structure. We describe
the current state of the art in this kind of analysis, with detailed
illustrations from on-going projects at Predictive Science. Separatrix
surfaces and quasi-separatrix layers form a structural skeleton of
magnetic configurations by dividing them into multiple components
with a simple topology. We discuss the principles and capabilities
of our techniques for analyzing the structural skeletons in erupting
configurations. In particular, we show how these techniques allow one:
(1) to identify erupting and non-erupting strands of the flux ropes; (2)
to determine the global topological flux cells in which such flux ropes
reside, and how they interact in successive eruptions; (3) to calculate
evolving magnetic fluxes for each component of these configurations;
(4) to relate certain structural features to observational features,
such as H-alpha flare ribbons, extreme-ultraviolet dimmings, and X-ray
sigmoids in solar eruptions. The ability to compare our results with
observations enables us to verify the accuracy of the MHD models and
to understand how the coronal magnetic field opens during eruptions.
Title: On the application of ensemble modeling techniques to improve
ambient solar wind models
Authors: Riley, Pete; Linker, Jon A.; Mikić, Zoran
Bibcode: 2013JGRA..118..600R
Altcode:
Ensemble modeling is a method of prediction based on the use of a
representative sample of possible future states. Global models of the
solar corona and inner heliosphere are now maturing to the point of
becoming predictive tools; thus, it is both meaningful and necessary
to quantitatively assess their uncertainty and limitations. In this
study, we apply simple ensemble modeling techniques as a first step
towards these goals. We focus on one relatively quiescent time period,
Carrington rotation 2062, which occurred during the late declining phase
of solar cycle 23. To illustrate and assess the sensitivity of the model
results to variations in boundary conditions, we compute solutions
using synoptic magnetograms from seven solar observatories. Model
sensitivity is explored using (1) different combinations of models,
(2) perturbations in the base coronal temperature (a free parameter in
one of the model approximations), and (3) the spatial resolution of the
numerical grid. We present variance maps, "whisker" plots, and "Taylor"
diagrams to summarize the accuracy of the solutions and compute skill
scores, which demonstrate that the ensemble mean solution outperforms
any of the individual realizations. Our results provide a baseline
against which future model improvements can be compared.
Title: The Structure and Dynamics of the Corona—Heliosphere
Connection
Authors: Antiochos, Spiro K.; Linker, Jon A.; Lionello, Roberto;
Mikić, Zoran; Titov, Viacheslav; Zurbuchen, Thomas H.
Bibcode: 2013mspc.book..169A
Altcode:
No abstract at ADS
Title: Pseudo-Streamer Magnetic Topologies in the 2010 August 1-2 CMEs
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J. A.;
Panasenco, O.
Bibcode: 2012AGUFMSH51A2211T
Altcode:
We upgrade our previous source-surface model of the background magnetic
field in the 2010 August 1-2 sympathetic CMEs to a more accurate
thermodynamic MHD model of the solar corona. For this new model,
we verify our earlier results on the structure of the large-scale
magnetic field, making a similar topological analysis of the field
as before. We identify the similarities and differences between the
two configurations, particularly, for the eruptive regions with three
pseudo-streamers that we have found before. The new study confirms
that all these pseudo-streamers indeed contain vertical separatrix
surfaces located between two adjacent disconnected coronal holes. Of
special interest to us are the magnetic null points and separator field
lines belonging to such separatrix surfaces. We reassert our earlier
hypothesis that magnetic reconnection at these nulls and separators
likely plays a key role in establishing a physical connection between
the successive eruptions observed by SDO and STEREO. The results
obtained provide further validation of our recent simplified MHD
model of sympathetic eruptions (Török et al. 2011). Work supported
by NASA's Heliophysics Theory and SR&T programs, and SHINE NSF
Grant AGS-1156119.
Title: The Interaction of Comet C/2011 W3 (Lovejoy) with the Global
Solar Corona: Insight and Perspectives from Realistic Thermodynamic
MHD Simulations
Authors: Downs, C.; Mikic, Z.; Linker, J. A.; Lionello, R.;
Schrijver, C.
Bibcode: 2012AGUFMSH13B2255D
Altcode:
On December 15-16 2011 the perihelion passage of sun-grazing comet
C-2011 W3 (Lovejoy) took it perilously close to the sun. Its voyage
through the low solar corona was readily observed by the EUV imagers
onboard the Solar Dynamics Observatory and the STEREO spacecraft
pair. During this time, EUV emission from the cometary tail underwent
several significant changes in both intensity and overall inclination
and shape, possibly an indication of rapidly changing ambient plasma
or magnetic field conditions. To study this phenomenon, we employ
a realistic 3D thermodynamic MHD model to obtain a self-consistent
picture of the global thermodynamic and magnetic structure of
the corona at this time. We use these simulations to examine the
ambient plasma parameters along the trajectory and place them in the
context of the multi-viewpoint observations, directly connecting the
important influence of the global magnetic field structure to comet
tail appearance and dynamics. By employing multiple simulations,
we demonstrate how such a unique set of observations can be used to
probe not only the comet-corona interaction, but also to constrain
properties of the coronal plasma and magnetic field.
Title: Prediction of the Solar Corona for the 2012 November 13 Total
Solar Eclipse
Authors: Mikic, Z.; Linker, J. A.; Downs, C.; Lionello, R.; Riley,
P.; Titov, V. S.; Torok, T.
Bibcode: 2012AGUFMSH33A2218M
Altcode:
It has become our tradition to predict the structure of the corona
prior to eclipses, using a magnetohydrodynamic (MHD) model based on
measurements of photospheric magnetic fields on the Sun. We plan to
continue this tradition by predicting the structure of the corona for
the November 13, 2012 total solar eclipse, using SDO/HMI photospheric
magnetic field data. We will predict the structure of the corona,
including images of polarization brightness, magnetic field line traces,
and images of simulated emission in EUV and X-rays. These images can
be compared directly with observations of the total eclipse, as well
as observations from SDO/AIA, Hinode/XRT, and STEREO/EUVI. Research
supported by NASA's Heliophysics Theory and Living With a Star Programs,
and NSF/FESD.
Title: Using multi-wavelength observations to constrain CME
simulations
Authors: Torok, T.; Mikic, Z.; Titov, V. S.; Linker, J. A.; Downs,
C.; Lionello, R.; Riley, P.
Bibcode: 2012AGUFMSH33E..01T
Altcode:
The steady growth of computing power now provides the possibility
to model coronal mass ejections (CMEs) at different levels of
complexity. Present CME simulations range from relatively simple
zero-beta calculations, which consider idealized configurations
to isolate the basic physical mechanisms at work in CMEs, to
semi-realistic "thermodynamic" MHD simulations of specific events
that allow us to confront the model results directly with the
observations. In this talk, we will discuss the respective benefits
of these different approaches. As an example, we will consider the
well-known sympathetic eruptions event on 2010, August 1, which our
group has been modeling using various degrees of approximation. In
particular, we will illustrate how we employed the observations (i)
to set up the respective initial magnetic configurations and (ii)
to validate the simulation results.
Title: Magnetohydrodynamic Simulations of Interplanetary Coronal
Mass Ejections
Authors: Lionello, R.; Downs, C.; Linker, J. A.; Torok, T.; Mikic, Z.
Bibcode: 2012AGUFMSH41B2117L
Altcode:
Accurately following the propagation of Interplanetary Coronal Mass
Ejections (ICME) is very important for determining space weather
conditions. These are known to impact the functioning of satellites
or create a dangerous environment for astronauts in orbit around the
Earth. Here we describe how we simulate with our MHD numerical model in
spherical coordinates the propagation of ICMEs from the critical point
to Earth and beyond. We first obtain the boundary conditions to apply at
the lower boundaries using the results of simulations of coronal mass
ejections. These are normally derived from the coronal version of our
own model, but any other model that can provide the components of the
magnetic field and the velocity, density, and pressure of the plasma
can be used. Then we calculate the propagation of the disturbance in
interplanetary space.
Title: The S-Web Hypothesis: Modeling and Observational Challenges
Authors: Linker, J. A.; Lionello, R.; Titov, V. S.; Mikic, Z.
Bibcode: 2012AGUFMSH52A..01L
Altcode:
The origin of the slow solar wind is controversial. A successful
theory must explain the plasma composition and angular extent of
the slow wind, as well as its frequent asymmetry with respect to the
heliospheric current sheet. Recently, a new idea has been put forward
for the origin of the slow wind, dubbed the "S-Web" model. The name
comes from high-resolution MHD calculations that have revealed that
coronal hole boundaries are not smooth, but are highly corrugated with a
web of separatrices and quasi-separatrix layers. These are regions that
are likely to be susceptible to interchange reconnection. In this talk
we describe the basics of this idea, how it may explain key features
of the slow solar wind, and further calculations/observational tests
that may help confirm or refute this idea. Work supported by NASA,
NSF, and AFOSR.
Title: Non-neutralized Electric Current Patterns in Solar Active
Regions: Origin of the Shear-generating Lorentz Force
Authors: Georgoulis, Manolis K.; Titov, Viacheslav S.; Mikić, Zoran
Bibcode: 2012ApJ...761...61G
Altcode: 2012arXiv1210.2919G
Using solar vector magnetograms of the highest available spatial
resolution and signal-to-noise ratio, we perform a detailed study
of electric current patterns in two solar active regions (ARs): a
flaring/eruptive and a flare-quiet one. We aim to determine whether
ARs inject non-neutralized (net) electric currents in the solar
atmosphere, responding to a debate initiated nearly two decades ago
that remains inconclusive. We find that well-formed, intense magnetic
polarity inversion lines (PILs) within ARs are the only photospheric
magnetic structures that support significant net current. More intense
PILs seem to imply stronger non-neutralized current patterns per
polarity. This finding revises previous works that claim frequent
injections of intense non-neutralized currents by most ARs appearing
in the solar disk but also works that altogether rule out injection of
non-neutralized currents. In agreement with previous studies, we also
find that magnetically isolated ARs remain globally current-balanced. In
addition, we confirm and quantify the preference of a given magnetic
polarity to follow a given sense of electric currents, indicating a
dominant sense of twist in ARs. This coherence effect is more pronounced
in more compact ARs with stronger PILs and must be of sub-photospheric
origin. Our results yield a natural explanation of the Lorentz force,
invariably generating velocity and magnetic shear along strong PILs,
thus setting a physical context for the observed pre-eruption evolution
in solar ARs.
Title: 2010 August 1-2 Sympathetic Eruptions. I. Magnetic Topology
of the Source-surface Background Field
Authors: Titov, V. S.; Mikic, Z.; Török, T.; Linker, J. A.;
Panasenco, O.
Bibcode: 2012ApJ...759...70T
Altcode: 2012arXiv1209.5797T
A sequence of apparently coupled eruptions was observed on 2010 August
1-2 by Solar Dynamics Observatory and STEREO. The eruptions were closely
synchronized with one another, even though some of them occurred at
widely separated locations. In an attempt to identify a plausible reason
for such synchronization, we study the large-scale structure of the
background magnetic configuration. The coronal field was computed from
the photospheric magnetic field observed at the appropriate time period
by using the potential field source-surface model. We investigate the
resulting field structure by analyzing the so-called squashing factor
calculated at the photospheric and source-surface boundaries, as well as
at different coronal cross-sections. Using this information as a guide,
we determine the underlying structural skeleton of the configuration,
including separatrix and quasi-separatrix surfaces. Our analysis
reveals, in particular, several pseudo-streamers in the regions where
the eruptions occurred. Of special interest to us are the magnetic
null points and separators associated with the pseudo-streamers. We
propose that magnetic reconnection triggered along these separators
by the first eruption likely played a key role in establishing the
assumed link between the sequential eruptions. The present work
substantiates our recent simplified magnetohydrodynamic model of
sympathetic eruptions and provides a guide for further deeper study
of these phenomena. Several important implications of our results for
the S-web model of the slow solar wind are also addressed.
Title: The Structure and Dynamics of the Corona—Heliosphere
Connection
Authors: Antiochos, Spiro K.; Linker, Jon A.; Lionello, Roberto;
Mikić, Zoran; Titov, Viacheslav; Zurbuchen, Thomas H.
Bibcode: 2012SSRv..172..169A
Altcode: 2011SSRv..tmp..371A; 2011SSRv..tmp..224A; 2011SSRv..tmp..148A;
2011SSRv..tmp...79A
Determining how the heliospheric magnetic field and plasma connect
to the Sun's corona and photosphere is, perhaps, the central problem
in solar and heliospheric physics. For much of the heliosphere,
this connection appears to be well understood. It is now generally
accepted that so-called coronal holes, which appear dark in X-rays
and are predominantly unipolar at the photosphere, are the sources
of quasi-steady wind that is generally fast, >500 km/s, but can
sometimes be slow. However, the connection to the Sun of the slow,
non-steady wind is far from understood and remains a major mystery. We
review the existing theories for the sources of the non-steady wind and
demonstrate that they have difficulty accounting for both the observed
composition of the wind and its large angular extent. A new theory is
described in which this wind originates from the continuous opening and
closing of narrow open field corridors in the corona, which give rise
to a web of separatrices (the S-Web) in the heliosphere. Note that
in this theory the corona—heliosphere connection is intrinsically
dynamic, at least for this type of wind. Support for the S-Web model
is derived from MHD solutions for the corona and wind during the time
of the August 1, 2008 eclipse. Additionally, we perform fully dynamic
numerical simulations of the corona and heliosphere in order to test
the S-Web model as well as the interchange model proposed by Fisk
and co-workers. We discuss the implications of our simulations for
the competing theories and for understanding the corona—heliosphere
connection, in general.
Title: MHD modeling of the solar corona: Progress and challenges
Authors: Linker, Jon; Mikic, Zoran; Lionello, Roberto; Riley, Pete;
Titov, Viacheslav; Torok, Tibor
Bibcode: 2012cosp...39.1090L
Altcode: 2012cosp.meet.1090L
The Sun and its activity is the ultimate driver of space weather at
Earth. This influence occurs not only via eruptive phenomena such as
coronal mass ejections, but also through the structure of the corona
itself, which forms the genesis of fast solar wind streams that trigger
recurrent geomagnetic activity. Coronal structure also determines the
connection of the ambient interplanetary magnetic field to CME-related
shocks and impulsive solar flares, and thus controls where solar
energetic particles propagate. In this talk we describe both the
present state of the art and new directions in coronal modeling for
both dynamic and slowly varying phenomena. We discuss the challenges to
incorporating these capabilities into future space weather forecasting
and specification models. Supported by NASA through the HTP, LWS,
and SR&T programs, by NSF through the FESD and CISM programs,
and by the AFOSR Space Science program.
Title: Corotating interaction regions during the recent solar minimum:
The power and limitations of global MHD modeling
Authors: Riley, Pete; Linker, Jon A.; Lionello, R.; Mikic, Z.
Bibcode: 2012JASTP..83....1R
Altcode:
The declining phase of solar activity cycle 23 has provided an
unprecedented opportunity to study the evolution and properties of
corotating interaction regions (CIRs) during unique and relatively
steady conditions. The absence of significant transient activity
has allowed modelers to test ambient solar wind models, but has also
challenged them to reproduce structure that was qualitatively different
than had been observed previously (at least within the space era). In
this study, we present and analyze global magnetohydrodynamic (MHD)
solutions of the inner heliosphere (from 1RS to 1 AU) for
several intervals defined as part of a Center for Integrated Space
weather Modeling (CISM) interdisciplinary campaign study, and, in
particular, Carrington rotation 2060. We compare in situ measurements
from ACE and STEREO A and B with the model results to illustrate both
the capabilities and limitations of current numerical techniques. We
show that, overall, the models do capture the essential structural
features of the solar wind for specific time periods; however, there
are times when the models and observations diverge. We describe, and,
to some extent assess the sources of error in the modeling chain from
the input photospheric magnetograms to the numerical schemes used to
propagate structure through the heliosphere, and speculate on how they
may be resolved, or at least mitigated in the future.
Title: Plasmoid Ejection at a Solar Total Eclipse
Authors: Koutchmy, S.; Bazin, C.; Berghmans, D.; De Groof, A.;
Druckmüller, M.; Tavabi, E.; Engell, A.; Filippov, B.; Golub, L.;
Lamy, Ph.; Linker, J.; Mikic, Z.; Mouette, J.; Nitschelm, Ch.; Seaton,
D.; Slemzin, V.
Bibcode: 2012EAS....55..223K
Altcode:
The existence of coronal plasmoids has been postulated for many years
in order to supply material to streamers and possibly to the solar
wind (SW). The W-L SoHO C2 Lasco coronagraph observations were made
under the 2.2 solar radii (R0) occulting disk to look at the ultimate
sources of the SW; EUV imagers are preferably devoted to the analysis
of the corona on and very near the solar disk. Here, in addition to
eclipse white-light (W-L) snapshots, we used the new SWAP space-borne
imager designed for the systematic survey of coronal activity in the
EUV lines near 17.4 nm, over a field of view (FOV) up to 2 R0. Using
summed and co-aligned images, the corona can then be evaluated for the
1st time up to the limit of this FOV. At the time of the July 11, 2010,
solar total eclipse a 20h continuous run of observations was collected,
including images taken during eclipse totality from several ground
observing locations where W-L data were collected. A plasmoid-like
off-limb event was followed using the SWAP summed
Title: Magnetic Topology of Pseudo-Streamers in the 2010 August 1-2
Eruption Events
Authors: Titov, Viacheslav S.; Mikic, Zoran; Torok, Tibor; Linker,
Jon A.; Panasenco, Olga
Bibcode: 2012shin.confE.160T
Altcode:
A sequence of apparently coupled eruptions was observed on 2010 August
1-2 by SDO and STEREO. The eruptions were closely synchronized, even
though some of them occurred very far from each other. Trying to
identify a plausible reason for such synchronization, we study the
large-scale structure of the background magnetic field. The latter
was computed from the photospheric magnetic field observed at the
appropriate time period by using the potential field source-surface
model.For the resulting configuration, we determine its structural
skeleton, which includes all separatrix and quasi-separatrix
surfaces. Analyzing them, we reveal three pseudo-streamers in the
regions where the eruptions occurred. Of special interest to us are
the magnetic null points and separator field lines associated with
these pseudo-streamers. We propose that magnetic reconnection at
such nulls and separators played likely a key role in establishing
the physical link between the successive eruptions. Work supported
by NASA's Heliophysics Theory and SR&T programs, and SHINE NSF
Grant AGS-1156119.
Title: Sympathetic Eruptive Events and Pseudostreamers
Authors: Panasenco, Olga; Titov, Viacheslav; Mikić, Zoran; Török,
Tibor; de Toma, Giuliana; Velli, Marco
Bibcode: 2012shin.confE.162P
Altcode:
Sequences of apparently coupled CMEs triggered by sympathetic eruptions
of solar filaments are usually observed when the initial coronal
magnetic configuration above the source region contains at least
one coronal pseudostreamer. We study in detail an example of such a
sympathetic event observed on 27-28 July 2011 by SDO and STEREO. This
involved five filaments and caused four individual filament eruptions
and one partial eruption. The eruptions were closely synchronized,
even though some occurred at widely separated locations. In an attempt
to identify a plausible reason of such a synchronization, we study the
large-scale structure of the background PFSS magnetic fields, computed
from the observed photospheric magnetic field (SDO/HMI) during the
appropriate time period. We investigate the magnetic connectivities in
these configurations by calculating and analyzing the distributions of
the so-called squashing factor at the photospheric and source-surface
boundaries, as well as other cross-sections at different heights. This
allows us to get a comprehensive understanding of the underlying
structural skeleton of the magnetic configuration. In particular,
our analysis reveals two pseudostreamer magnetic configurations in the
region where the eruptions occurred. Of special interest to us are the
magnetic null points and separators located at the intersection of the
separatrix domes and curtains of the pseudostreamers. We assume that
magnetic reconnection induced by the first eruption at these locations
played likely a major role in establishing the postulated link between
the different eruptions in sequence. The close relationship between the
sympathetic eruptions and pseudostreamer configurations are supported
by a statistical study covering the SDO era (2010-2012).
Title: Global Thermodynamic MHD Modeling of the Solar Corona in the
Context of SDO/AIA Observations.
Authors: Downs, Cooper James; Linker, Jon A.; Mikic, Zoran; Lionello,
Roberto; Riley, Pete
Bibcode: 2012shin.confE..98D
Altcode:
Realistic magnetohydrodynamic (MHD) models can serve as powerful
testbeds for exploring our understanding of magnetic and thermodynamic
processes in the solar corona. An important aspect in their development
is the use of observations to characterize model results. In
this context we investigate the comparison of observations from
the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) to a thermodynamic MHD model of the global corona
(Lionello et al. 2009), with emphasis on exploiting the unique
multi-spectral information available from the AIA observing program. We
discuss the implications towards modeling multi-temperature magnetic
structures observed in the low corona, and focus on the sensitivity
of the AIA observables with respect to the choice of coronal heating
parameterizations and magnetic boundary data.Research Supported by
NASA and NSF.
Title: How Well Do We Understand Solar-Heliospheric Connectivity?
Authors: Linker, Jon A.; Titov, Viacheslav; Lionello, Roberto; Mikic,
Zoran; Cohen, Christina
Bibcode: 2012shin.confE..17L
Altcode:
In the classic picture of the heliospheric magnetic field, magnetic
field lines move outward from the Sun and travel eastward along an
approximately 45 degree spiral at 1 AU (the exact value determined by
the rotation rate of the Sun and the speed of the solar wind). Solar
energetic particles propagate along the magnetic field, so when
there is a relatively compact source of particles it should be seen
in a relatively confined portion of interplanetary space. However, a
recent validation study (Macneice et al. 2011) showed that a range of
coronal-heliospheric models performed poorly in predicting the source
of confined SEP events. Recent observations from the STEREO and ACE
spacecraft have also shown that Helium-3 rich SEP events can be observed
over a wide range of longitude, even though their origin appears to
be localized to a single active region.The S-Web Model (Antiochos et
al. 2011, Linker et al. 2011, Titov et al. 2011) hypothesizes that
a web of separators and quasi-separatrix layers are present in the
coronal magnetic field, and that this web plays an important role in
the origin of the slow solar wind. In this presentation, we explore the
role that the S-web may play in SEP propagation. We show examples where
magnetic field lines originating in or near an active region may have
access to a wide range of connectivity, and this could allow particles
to propagate in a much larger range of latitudes and longitudes than
than would usually be expected.
Title: Global energy diagnostics, current sheet formation and
reconnection outflow jets in a thermodynamic 3D MHD CME simulation
Authors: Reeves, Kathy; Mikić, Zoran; Linker, Jon; Török, Tibor;
Murphy, Nick
Bibcode: 2012shin.confE..40R
Altcode:
We model a CME using a 3D numerical MHD code that includes coronal
heating, thermal conduction and radiative cooling in the energy
equation. We first develop a global coronal solution (from 1 to 20
Rs) to serve as the initial condition for the CME simulation. The
magnetic flux distribution at 1 Rs consists of a local subsurface
dipole superimposed on global dipole, to mimic the presence of an
active region within the global corona. The resulting configuration
has solar wind emanating from the open field regions, dense plasma in
the streamer belt, and hot plasma in the active region. We introduce
transverse electric fields near the neutral line in the active
region to form a flux rope, then a converging flow is imposed that
causes the eruption. We examine the global energy budget for this
simulated eruption, including the magnetic, kinetic, internal and
gravitational potential energies, coronal heating, ohmic heating,
flow of Poynting flux across the simulation boundaries, and losses due
to radiation. These diagnostic are useful in assessing whether such
simulations reproduce the characteristics of CME observations. We also
follow the formation and evolution of the current sheet and reconnection
outflow jets in this model.
Title: Modeling the Large-Scale Structure of the Solar Corona and
Inner Heliosphere during Carrington Rotations 2058 and 2062
Authors: Riley, Pete; Linker, Jon; Lionello, Roberto; Mikic, Zoran;
Downs, Cooper
Bibcode: 2012shin.confE.205R
Altcode:
Accurate global MHD models can help us understand the basic physical
processes that produce and modulate structure in the solar corona and
inner heliosphere. In turn, when these models can be routinely run and
accurately match in situ observations at Earth, they become capable
of near-term ( 4 day) space weather forecasting. In this poster,
we summarize the current status of our ambient solar wind modeling
effort. The model, CORHEL, is driven by the observed magnetic field
in the photosphere, and aims to capture the state of the corona and
inner heliosphere during specific time periods of interest at the
highest spatial resolutions possible. By including energy transport
processes, such as coronal heating, anisotropic thermal conduction,
and radiative losses, we can reproduce EUV and X-ray emission and
make meaningful comparisons with observations. We compare our model
results with both remote solar observations and in situ measurements
for Carrington rotations 2058 and 2062 and assess both the advantages
and limitations of our techniques. In particular, we employ ensemble
modeling methodologies, varying both the input boundary conditions and
model formalisms, to quantitatively assess the uncertainty in the model
results. We discuss how future improvements both to the model and the
input boundary conditions may result in better matches with observations
and a greater understanding of the mechanisms that give rise to solar
wind structure. Research supported by NASA, NSF, and AFOSR.
Title: Modeling the global structure of the heliosphere during the
recent solar minimum: Model improvements and unipolar streamers
Authors: Riley, Pete; Stevens, Michael; Linker, Jon A.; Lionello,
Roberto; Mikic, Zoran; Luhmann, Janet G.
Bibcode: 2012AIPC.1436..337R
Altcode:
The recent solar minimum, marking the end of solar cycle 23, has been
unique in a number of ways. In particular, the polar photospheric
flux was substantially weaker, coronal holes were notably smaller,
and unipolar streamers were considerably more prevalent than previous
minima. To understand the origins of some of these phenomena, we have
computed global solutions using a three-dimensional, time-dependent MHD
model of the solar corona and heliosphere. In this report, we present
a brief overview of a selection of model results, illustrating: (1)
how observations are being used to better constrain model properties;
and (2) how the model results can be applied to understanding complex
coronal and interplanetary phenomena, and, specifically, unipolar
streamers.
Title: Are Polar Field Magnetic Flux Concentrations Responsible for
Missing Interplanetary Flux?
Authors: Linker, Jon A.; Downs, C.; Mikic, Z.; Riley, P.; Henney,
C. J.; Arge, C. N.
Bibcode: 2012AAS...22041101L
Altcode:
Magnetohydrodynamic (MHD) simulations are now routinely used to produce
models of the solar corona and inner heliosphere for specific time
periods. These models typically use magnetic maps of the photospheric
magnetic field built up over a solar rotation, available from a number
of ground-based and space-based solar observatories. The line-of-sight
field at the Sun's poles is poorly observed, and the polar fields in
these maps are filled with a variety of interpolation/extrapolation
techniques. These models have been found to frequently underestimate
the interplanetary magnetic flux (Riley et al., 2012, in press, Stevens
et al., 2012, in press) near the minimum part of the cycle unless
mitigating correction factors are applied. Hinode SOT observations
indicate that strong concentrations of magnetic flux may be present at
the poles (Tsuneta et al. 2008). The ADAPT flux evolution model (Arge
et al. 2010) also predicts the appearance of such concentrations. In
this paper, we explore the possibility that these flux concentrations
may account for a significant amount of magnetic flux and alleviate
discrepancies in interplanetary magnetic flux predictions. Research
supported by AFOSR, NASA, and NSF.
Title: A New Perspective of Coronal-Loop EUV Emissions
Authors: Mok, Yung; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2012AAS...22020206M
Altcode:
The formation mechanism of coronal loops and why they appear to
have uniform cross section have been controversial. In general, the
flux tubes of the active-region magnetic field expand in the corona,
especially at locations where the field is nearly potential. Because
the actual magnetic field, modeled in 3D based on a magnetogram,
is complicated, the attempt to extract the underlying loop physics
might have been hindered by the complexity of the field. We go back
to basic by using a simple quadrupole field structure and applying a
heating model in which heat is deposited mostly near the footpoint. Our
3D simulation demonstrates that this simple structure is able to
reproduce loop-like structures in EUV. Although the flux tubes expand
in the corona as expected, the EUV loops have remarkably uniform cross
sections. The dynamic properties of these synthetic loops are to be
compared with the observations. Work supported by Heliospheric
Theory Program of NASA
Title: Global Thermodynamic MHD Modeling of the Solar Corona in the
Context of SDO/AIA Observations.
Authors: Downs, Cooper; Linker, J. A.; Mikic, Z.; Lionello, R.;
Riley, P.
Bibcode: 2012AAS...22020714D
Altcode:
Realistic magnetohydrodynamic (MHD) models can serve as powerful
testbeds for exploring our understanding of magnetic and thermodynamic
processes in the solar corona. An important aspect in their development
is the use of observations to characterize model results. In
this context we investigate the comparison of observations from
the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) to a thermodynamic MHD model of the global corona
(Lionello et al. 2009), with emphasis on exploiting the unique
multi-spectral information available from the AIA observing program. We
discuss the implications towards modeling multi-temperature magnetic
structures observed in the low corona, and focus on the sensitivity
of the AIA observables with respect to the choice of coronal heating
parameterizations and magnetic boundary data. Research Supported
by NASA and NSF.
Title: A First Look at Magnetic Field Data Products from SDO/HMI
Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai,
T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat,
S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares,
C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.;
DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk,
S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka,
K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.;
Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić,
Z.; Riley, P.; Wu, S. T.
Bibcode: 2012ASPC..455..337L
Altcode:
The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011)
is one of the three instruments aboard the Solar Dynamics Observatory
(SDO) that was launched on February 11, 2010 from Cape Canaveral,
Florida. The instrument began to acquire science data on March 24. The
regular operations started on May 1. HMI measures the Doppler velocity
and line-of-sight magnetic field in the photosphere at a cadence of
45 seconds, and the vector magnetic field at a 135-second cadence,
with a 4096× 4096 pixels full disk coverage. The vector magnetic
field data is usually averaged over 720 seconds to suppress the p-modes
and increase the signal-to-noise ratio. The spatial sampling is about
0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which
has a Landé factor of 2.5. These data are further used to produce
higher level data products through the pipeline at the HMI-AIA Joint
Science Operations Center (JSOC) - Science Data Processing (Scherrer et
al. 2011) at Stanford University. In this paper, we briefly describe the
data products, and demonstrate the performance of the HMI instrument. We
conclude that the HMI is working extremely well.
Title: Observations and simulations of the sympathetic eruptions on
2010 August 1
Authors: Torok, T.; Mikic, Z.; Panasenco, O.; Titov, V. S.; Reeves,
K. K.; Velli, M.; Linker, J. A.; de Toma, G.
Bibcode: 2012EGUGA..14.3270T
Altcode:
During the rise of the new solar cycle, the Sun has produced a number
of so-called sympathetic eruptions, i.e., eruptions that occur close in
time in different source regions. While it has become clear in recent
years that in many of such events the individual eruptions must be
magnetically connected, the exact nature of these connections is not
yet understood. A particularly beautiful case, which consisted of half
a dozen individual eruptions, was observed by STEREO and SDO on 2010
August 1. Here we focus on a subset of two large, consecutive filament
eruptions that were preceded by a nearby CME. We first summarize the
main features of these events and then present 3D MHD simulations
that were designed to model such a chain of eruptions. The simulations
suggest that the two filament eruptions were triggered by two successive
reconnection events, each of which was induced by the previous eruption,
and thus provide a new mechanism for sympathetic eruptions.
Title: Global MHD Models of the Corona and Solar Wind
Authors: Mikic, Z.; Linker, J. A.; Lionello, R.; Riley, P.; Titov,
V. S.; Torok, T.
Bibcode: 2012decs.confE..85M
Altcode:
Magnetohydrodynamic (MHD) models are useful in understanding the
properties of the global solar corona. They typically use measured
photospheric magnetic fields and an empirical specification of coronal
heating. Comparisons of simulated EUV and X-ray emission from such
models with observations (such as SOHO/EIT, Hinode/XRT, STEREO/EUVI, and
SDO/AIA) can provide a tight constraint on coronal heating models. We
will describe how these models can be used to improve our understanding
of the process that heats the corona.
Title: Connecting the photosphere to the corona : Reconstructing
the Solar Coronal Magnetic Field
Authors: Amari, T.; Delyon, F.; Alauzet, F.; Canou, A.; Mikic, Z.;
Aly, J. J.; Solis Team; Stanford Sdo/Hmi Team
Bibcode: 2012decs.confE..50A
Altcode:
The low solar corona is dominated by the magnetic field which is
created inside the sun by a dynamo process and then emerges into
the atmosphere. This magnetic field plays an important role in most
structures and phenomena observed at various wavelengths such as
prominences, small and large scale eruptive events, and continuous
heating of the plasma, and therefore it is important to understand
its three-dimensional properties in order to elaborate efficient
theoretical models. Unfortunately, the magnetic field is difficult
to measure locally in the hot and tenuous corona. But this can be
done at the level of the cooler and denser photosphere, and several
instruments with high resolution vector magnetographs are currently
available (THEMIS, Imaging Vector Magnetograph (IVM), the Advanced
Stokes Polarimeter (ASP)), SOLIS, HINODE , Solar Dynamics Observatory
(SDO), or will be shortly available and future programmed missions
such as , SOLAR-ORBITER. This has lead solar physicists to develop
an approach which consists in reconstructing the coronal magnetic
field from boundary data given on the photosphere. We will present
our recent progress and results to solve this problem at the scale of
active regions or larger ones such as full disk or synoptic scales,
for which the large amount of data as well as their sparsity on the
solar disk, require to develop particular strategies. We will also
illustrate the interest of the reconstruction for characterizing
the magnetic environments of prominences, emerging sub-photospheric
structures and the pre-eruptive ones.
Title: Coronal Mass Ejection Initiation by Converging Photospheric
Flows: Toward a Realistic Model
Authors: Amari, T.; Aly, J. -J.; Luciani, J. -F.; Mikic, Z.; Linker, J.
Bibcode: 2011ApJ...742L..27A
Altcode:
In the context of coronal mass ejections triggering, we reconsider
the class of models in which the evolution of an active region
(AR) is driven by imposed boundary motions converging toward the
polarity inversion line (PIL). We introduce a new model problem in
which there is a large-scale flow with a diverging structure on the
photosphere. This flow is reminiscent of that of the well-known moat
flow around each of the two spots of a bipolar AR and transports only
part of the magnetic flux toward the PIL. It is thus more compatible
with observations than the one used in our previous study, which forced
the whole positive and negative polarity parts of the AR approaching
each other. We also include a diffusion term associated with small-scale
turbulent photospheric motions, but keep the associated diffusivity at
a low value in the particular study described here. We show that the
evolution of an initial sheared force-free field first leads to the
formation of a twisted flux rope which stays in equilibrium for some
time. Eventually, however, the configuration suffers a global disruption
whose underlying mechanism is found by energetic considerations to
be nonequilibrium. It begins indeed when the magnetic energy becomes
of the order of the energy of an accessible partially open field. For
triggering an eruption by converging flows, it is thus not necessary
to advect the whole AR toward the PIL, but only its central part.
Title: MHD Modeling of the Sympathetic Eruptions Observed on August
1, 2010
Authors: Mikic, Z.; Torok, T.; Titov, V. S.; Linker, J. A.; Lionello,
R.; Riley, P.
Bibcode: 2011AGUFMSH41B..04M
Altcode:
The multiple solar eruptions observed by SDO on August 1, 2010 present a
special challenge to theoretical models of CME initiation. SDO captured
in detail a remarkable chain of sympathetic eruptions that involved
the entire visible hemisphere of the Sun (Schrijver et al. 2011). It
consisted of several flares and six filament eruptions/CMEs, and
triggered a geomagnetic storm on August 3 (de Toma et al. 2010). This
series of eruptions was also observed by the two STEREO spacecraft. This
collection of observations presents a unique opportunity to understand
sympathetic eruptions theoretically. We will present 3D MHD simulations
of these events that have helped us to understand the possible
mechanisms by which the various filament eruptions/CMEs may be linked,
with particular emphasis on the global topology of the coronal magnetic
field in which these structures are embedded.
Title: The Unusual Minimum Preceding Cycle 24: What MHD Models Reveal
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.; Titov,
V. S.
Bibcode: 2011AGUFMSH23D..04L
Altcode:
The mimimum preceding solar cycle 24 is characterized by unusual
properties when compared with the previous cycle and other space age
minima. These features include many more spotless days, weaker polar
magnetic field strengths, weaker interplanetary magnetic fields, and
persistent isolated equatorial coronal holes. We use high-resolution
3D MHD simulations of the corona to investigate coronal structure
during the deep minimum. We compare these results with simulations of
the corona for the minimum prior to solar cycle 23, to elucidate the
differences between the minima. We will also discuss how the recent
unusual minimum provides opportunities to test our understanding
of the underlying processes that produce the corona and solar wind,
particularly the origin of the slow solar wind. Research supported by
NASA and NSF.
Title: Global MHD Modeling of the Solar Corona and Inner Heliosphere
for the Whole Heliosphere Interval
Authors: Riley, P.; Lionello, R.; Linker, J. A.; Mikic, Z.; Luhmann,
J.; Wijaya, J.
Bibcode: 2011SoPh..274..361R
Altcode:
In an effort to understand the three-dimensional structure of the
solar corona and inner heliosphere during the Whole Heliosphere
Interval (WHI), we have developed a global magnetohydrodynamics
(MHD) solution for Carrington rotation (CR) 2068. Our model,
which includes energy-transport processes, such as coronal heating,
conduction of heat parallel to the magnetic field, radiative losses,
and the effects of Alfvén waves, is capable of producing significantly
better estimates of the plasma temperature and density in the corona
than have been possible in the past. With such a model, we can
compute emission in extreme ultraviolet (EUV) and X-ray wavelengths,
as well as scattering in polarized white light. Additionally, from
our heliospheric solutions, we can deduce magnetic-field and plasma
parameters along specific spacecraft trajectories. In this paper, we
present a general analysis of the large-scale structure of the solar
corona and inner heliosphere during WHI, focusing, in particular, on i)
helmet-streamer structure; ii) the location of the heliospheric current
sheet; and iii) the geometry of corotating interaction regions. We
also compare model results with i) EUV observations from the EIT
instrument onboard SOHO; and ii) in-situ measurements made by the
STEREO-A and B spacecraft. Finally, we contrast the global structure of
the corona and inner heliosphere during WHI with its structure during
the Whole Sun Month (WSM) interval. Overall, our model reproduces the
essential features of the observations; however, many discrepancies
are present. We discuss several likely causes for them and suggest
how model predictions may be improved in the future.
Title: Comparing MHD Simulations of the Solar Corona and the Solar
Wind with Data
Authors: Lionello, R.; Linker, J. A.; Mikic, Z.; Riley, P.; Titov,
V. S.; Torok, T.
Bibcode: 2011AGUFMSH41B..02L
Altcode:
Our global three-dimensional magnetohydrodynamic (MHD) model of the
solar corona and the solar wind has been extensively used to model
the properties of the magnetic field and of the plasma, from Sun
to Earth and beyond. The key observational input to the model is the
incorporation of observed photospheric magnetic fields into the boundary
conditions. We have studied the geometrical and topological properties
of the magnetic field (e.g., the location and evolution of corona holes,
the reproduction of streamer structure, the location of the heliospheric
current sheet, etc.) and its dynamical reconfiguration (e.g., eruptions
and CMEs propagation). Direct comparison with observations have been
made in the corona by calculation of emission in several EUV and X-ray
bands, both for loops and the global corona. We have also compared the
simulated speed, density, temperature, and magnetic field in the solar
wind with in situ observations. We will discuss the insights obtained
on the strengths and limitations of the models from these comparisons.
Title: How do Heliospheric Remote-Sensing Observations Limit Magnetic
Flux Rope Models?
Authors: Riley, P.; Torok, T.; Mikic, Z.; Linker, J. A.; Lionello,
R.; Titov, V. S.
Bibcode: 2011AGUFMSH24A..02R
Altcode:
In-situ measurements of coronal mass ejecta (CMEs) display a range of
properties, only some of which can be accounted for by current global
MHD models. In fact, first-principle models that include the initiation
and eruption of the ejecta necessarily produce well-defined flux ropes,
whereas only a fraction of CMEs observed in-situ appear to contain a
flux rope. In this talk, we summarize our current understanding of
the observed properties of interplanetary flux ropes and ejecta in
general. We explore ideas that the dichotomy between CMEs and flux
ropes might be due to: (1) an observational selection effect, that is,
all CMEs do in fact contain flux ropes and that the trajectory of the
spacecraft through the event is what determines whether a flux rope is
also encountered; (2) interactions of an erupting flux rope with itself
or between neighboring flux ropes to produce complex structures in which
the flux-rope structure has been significantly modified or destroyed;
(3) an evolutionary process, such as relaxation to a low plasma-beta
state, which governs whether a flux rope is present or not; or (4)
the existence of two (or more) intrinsic mechanisms for producing CMEs,
some of which produce flux ropes and some that do not. To assess these
ideas, we compare model results with a selection of CMEs observed by
the Ulysses, ACE, and STEREO spacecraft.
Title: Advances in Modeling the initiation and evolution of CMEs
through the Solar WInd
Authors: Riley, P.; Mikic, Z.; Linker, J. A.; Torok, T.; Lionello,
R.; Titov, V. S.
Bibcode: 2011AGUFMSH53C..05R
Altcode:
Over the last decade, several factors have led to remarkable gains
in our ability to realistically model a coronal mass ejection (CME)
all the way from the solar surface to 1 AU, or beyond. First,
global models of the ambient solar corona and inner heliosphere
have improved dramatically. The algorithms have transitioned from
simple polytropic prescriptions to rich thermodynamic models that
can reproduce the essential features of remote solar observations and
in-situ measurements. Second, theories of CME initiation, and their
implementation into numerical models, have developed to the point
that a range of complex mechanisms can now be simulated with great
fidelity. Third, the original serial codes are now fully parallelized
allowing them to recruit thousands of processors, and with this,
the ability to simulate events on unprecedented temporal and spatial
scales. And fourth, successive NASA-led missions are returning ever-more
resolved and accurate photospheric magnetic field observations from
which boundary conditions can be derived. In this talk, we show how
these factors have allowed us to produce event-specific simulations
that provide genuine insight into the initiation and evolution of
CMEs, and contrast these results with what was "state-of-the-art"
only 10 years ago. We close by speculating on what the next advances
in global CME models might be.
Title: Magnetic Topology of the Sympathetic CMEs Observed on 27 July
2011 and 1 August 2010
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J. A.;
Panasenco, O.
Bibcode: 2011AGUFMSH43B1949T
Altcode:
Two fascinating sequences of apparently coupled CMEs were observed
on 27-28 July 2011 and 1-2 August 2010 by SDO and STEREO. The latter
sequence has recently been described at length by Schrijver &
Title (2011). In both CME sequences, the individual eruptions were
closely synchronized with one another, even though some of them
occurred at widely separated locations. In an attempt to identify a
plausible reason of such a synchronization, we study the large-scale
structure of the background PFSS magnetic fields that were computed
from the observed photospheric magnetic field at the appropriate
time period. We investigate the magnetic connectivities in these
configurations by calculating and analyzing the distributions of the
so-called squashing factor at the photospheric and source-surface
boundaries, as well as at different cross-sections. This allows us
to get a comprehensive understanding of the underlying structural
skeleton of the magnetic configuration. In particular, our analysis
reveals several pseudo-streamers in the regions where the eruptions
occurred. Of special interest to us are the magnetic null points and
separators located at the intersection of the separatrix domes and
curtains of the pseudo-streamers. We assume that magnetic reconnection
induced by the first eruption at these locations played likely a major
role in establishing the postulated link between the eruptions in both
CME sequences. Our recent simplified MHD model of sympathetic eruptions
supports this assumption (Török et al. 2011). In the present study,
we try to further verify it by comparing the background magnetic
topologies of the two sequences of CMEs. Work supported by NASA and
the Center for Integrated Space Weather Modeling (an NSF Science and
Technology Center).
Title: Observations of the White Light Corona from Solar Orbiter
and Solar Probe Plus
Authors: Howard, R. A.; Thernisien, A. F.; Vourlidas, A.; Plunkett,
S. P.; Korendyke, C. M.; Sheeley, N. R.; Morrill, J. S.; Socker,
D. G.; Linton, M. G.; Liewer, P. C.; De Jong, E. M.; Velli, M. M.;
Mikic, Z.; Bothmer, V.; Lamy, P. L.
Bibcode: 2011AGUFMSH43F..06H
Altcode:
The SoloHI instrument on Solar Orbiter and the WISPR instrument on Solar
Probe+ will make white light coronagraphic images of the corona as the
two spacecraft orbit the Sun. The minimum perihelia for Solar Orbiter
is about 60 Rsun and for SP+ is 9.5 Rsun. The wide field of view of the
WISPR instrument (about 105 degrees radially) corresponds to viewing
the corona from 2.2 Rsun to 20 Rsun. Thus the entire Thomson hemisphere
is contained within the telescope's field and we need to think of
the instrument as being a traditional remote sensing instrument and
then transitioning to a local in-situ instrument. The local behavior
derives from the fact that the maximum Thomson scattering will favor
the electron plasma close to the spacecraft - exactly what the in-situ
instruments will be sampling. SoloHI and WISPR will also observe
scattered light from dust in the inner heliosphere, which will be an
entirely new spatial regime for dust observations from a coronagraph,
which we assume to arise from dust in the general neighborhood of about
half way between the observer and the Sun. As the dust grains approach
the Sun, they evaporate and do not contribute to the scattering. A
dust free zone has been postulated to exist somewhere inside of 5 Rsun
where all dust is evaporated, but this has never been observed. The
radial position where the evaporation occurs will depend on the
precise molecular composition of the individual grains. The orbital
plane of Solar Orbiter will gradually increase up to about 35 degrees,
enabling a very different view through the zodiacal dust cloud to test
the models generated from in-ecliptic observations. In this paper we
will explore some of the issues associated with the observation of
the dust and will present a simple model to explore the sensitivity
of the instrument to observe such evaporations.
Title: Formation of the current sheet in a coronal streamer
Authors: Abbo, Lucia; Antonucci, Ester; Lionello, Roberto; Mikić,
Zoran; Riley, Pete
Bibcode: 2011arXiv1111.2711A
Altcode:
The present work is on the study of a coronal streamer observed
in March 2008 at high spectral and spatial resolution by the
Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO. On the
basis of a spectroscopic analysis of the O VI doublet, the solar wind
plasma parameters are inferred in the extended corona. The analysis
accounts for the coronal magnetic topology, extrapolated through
a 3D magneto-hydrodynamic model. The results of the analysis show
indications on the formation of the current sheet, one of the source
regions of the slow coronal wind.
Title: A Model for Magnetically Coupled Sympathetic Eruptions
Authors: Török, T.; Panasenco, O.; Titov, V. S.; Mikić, Z.; Reeves,
K. K.; Velli, M.; Linker, J. A.; De Toma, G.
Bibcode: 2011ApJ...739L..63T
Altcode: 2011arXiv1108.2069T
Sympathetic eruptions on the Sun have been observed for several decades,
but the mechanisms by which one eruption can trigger another remain
poorly understood. We present a three-dimensional MHD simulation that
suggests two possible magnetic trigger mechanisms for sympathetic
eruptions. We consider a configuration that contains two coronal flux
ropes located within a pseudo-streamer and one rope located next to
it. A sequence of eruptions is initiated by triggering the eruption of
the flux rope next to the streamer. The expansion of the rope leads
to two consecutive reconnection events, each of which triggers the
eruption of a flux rope by removing a sufficient amount of overlying
flux. The simulation qualitatively reproduces important aspects of the
global sympathetic event on 2010 August 1 and provides a scenario for
the so-called twin filament eruptions. The suggested mechanisms are
also applicable for sympathetic eruptions occurring in other magnetic
configurations.
Title: A model for sympathetic eruptions
Authors: Torok, Tibor; Panasenco, O.; Titov, V. S.; Mikic, Z.; Velli,
M.; Linker, J.; De Toma, G.
Bibcode: 2011shin.confE.125T
Altcode:
Apart from single eruptions originating in localized source regions,
the Sun sometimes produces so-called sympathetic events, which consist
of several individual eruptions occurring almost simultaneously
in different source regions. The close temporal correlation of the
individual eruptions in such events indicates a causal link between
them, but the mechanisms by which one eruption can trigger another
one remain largely a mystery. A particularly beautiful example
of a global sympathetic event was recently observed by the Solar
Dynamics Observatory (SDO) on 1 August 2010. It included a small
filament eruption and CME that was shortly after followed by the
nearby subsequent eruptions of two large adjacent (twin) filaments,
indicating that these three eruptions were physically connected. A
coronal potential field extrapolation reveals that the twin filaments
were located in the lobes of a so-called pseudostreamer prior to
their eruptions. Here we present a 3D MHD simulation of the
successive eruption of two magnetic flux ropes in such a pseudostreamer
configuration. The two eruptions are triggered by the simulated eruption
of a third flux rope in the vicinity of the pseudostreamer. The
simulation qualitatively reproduces the CME and subsequent twin
filament eruption on 1 August 2010 and suggests that these events
were indeed physically connected. Furthermore, it provides a generic
scenario for the frequently observed twin filament eruptions in coronal
pseudostreamers and suggests a mechanism by which such eruptions can
be triggered in the first place. Our results thus provide an important
step for a better understanding of sympathetic eruptions.
Title: Magnetic Topology Diagnostics and the Slow Solar Wind
Authors: Lionello, Roberto; Linker, J. A.; Mikic, Z.; Titov, V. S.
Bibcode: 2011shin.confE.156L
Altcode:
In a 3D MHD simulation of the solar corona, we advect two magnetic
bipoles into and out of a coronal hole. This causes a complex
reconfiguration of the magnetic field, with field lines that open up,
close down, or undergo interchange reconnection. We examine magnetic
topology diagnostics and look for indications of slow wind formation.
Title: Structural Skeleton of the Background Magnetic Field During
Sympathetic Eruptions on 1-2 August 2010
Authors: Titov, Viacheslav S.; Mikić, Zoran; Török, Tibor; Linker,
Jon A.
Bibcode: 2011shin.confE.131T
Altcode:
The Solar Dynamics Observatory observed on 1-2 August 2010 an
interesting sequence of coronal mass ejections (CMEs) (Schrijver &
Title, 2011). These CMEs were closely synchronized with one another,
even though some of them occurred at remote locations. Therefore,
it is tempting to assume that these events were causally linked. In
an attempt to identify a plausible reason of such a link, we study
a large-scale structure of the background magnetic field that has
been computed from the observed photospheric magnetic field at the
appropriate time period. For this purpose, we investigate the respective
magnetic connectivity in the obtained configuration by calculating
and analyzing the distributions of the so-called squashing factor at
the boundaries as well as at different cross-sections. This allows
us to get a comprehensive understanding of the underlying structural
skeleton of the magnetic configuration. In particular, we have found
that five of the six erupting flux ropes were located inside the domes
of three pseudostreamers adjoint to the active region AR 11094. The
stalks of the pseudostreamers passed along the fan separatrix surfaces
emanating upward from the respective magnetic null points. We assume
that magnetic reconnection at these null points played likely a major
role in establishing a hypothetical causal link between the indicated
CMEs. The obtained topological framework provides a solid guide for
further numerical modeling and analysis of the observational data of
these events. Work supported by NASA and the Center for Integrated
Space Weather Modeling (an NSF Science and Technology Center).
Title: The S-Web Hypothesis and the Slow Solar Wind
Authors: Linker, Jon A.; Lionello, Roberto; Titov, Viacheslav S.;
Mikic, Zoran; Antiochos, Spiro
Bibcode: 2011shin.confE.160L
Altcode:
The origin of the slow solar wind is controversial. A successful theory
must explain the plasma composition and angular extent of the slow wind,
as well as its frequent asymmetry with respect to the heliospheric
current sheet. Recently, a new idea has been put forward for the
origin of the slow wind, dubbed the S-Web model. The name comes from
high-resolution MHD calculations that have revealed that coronal hole
boundaries are not smooth, but are highly corrugated with a web of
separatrices and quasi-separatrix layers. These are regions that are
likely to be susceptible to interchange reconnection. In this talk we
describe the origin of this idea, how it may explain key features of the
slow solar wind, and further calculations/observational tests that may
help confirm or refute this idea. Work supported by NASA and NSF.
Title: Coronal Plumes in the Fast Solar Wind
Authors: Velli, Marco; Lionello, Roberto; Linker, Jon A.; Mikić, Zoran
Bibcode: 2011ApJ...736...32V
Altcode:
The expansion of a coronal hole filled with a discrete number of
higher density coronal plumes is simulated using a time-dependent
two-dimensional code. A solar wind model including an exponential
coronal heating function and a flux of Alfvén waves propagating both
inside and outside the structures is taken as a basic state. Different
plasma plume profiles are obtained by using different scale heights for
the heating rates. Remote sensing and solar wind in situ observations
are used to constrain the parameter range of the study. Time dependence
due to plume ignition and disappearance is also discussed. Velocity
differences of the order of ~50 km s-1, such as those
found in microstreams in the high-speed solar wind, may be easily
explained by slightly different heat deposition profiles in different
plumes. Statistical pressure balance in the fast wind data may be
masked by the large variety of body and surface waves which the higher
density filaments may carry, so the absence of pressure balance in the
microstreams should not rule out their interpretation as the extension
of coronal plumes into interplanetary space. Mixing of plume-interplume
material via the Kelvin-Helmholtz instability seems to be possible
within the parameter ranges of the models defined here, only at large
distances from the Sun, beyond 0.2-0.3 AU. Plasma and composition
measurements in the inner heliosphere, such as those which will become
available with Solar Orbiter and Solar Probe Plus, should therefore
definitely be able to identify plume remnants in the solar wind.
Title: 3d Mhd Simulation Of Sympathetic Eruptions On 1 August 2010
Authors: Torok, Tibor; Panasenco, O.; Titov, V.; Mikic, Z.; Reeves,
K.; Velli, M.; Linker, J.; de Toma, G.
Bibcode: 2011SPD....42.0908T
Altcode: 2011BAAS..43S.0908T
Apart from single eruptions originating in localized source regions, the
Sun sometimes produces so-called sympathetic events, which consist of
several individual eruptions occurring almost simultaneously
in different source regions. The close temporal vicinity of the
individual eruptions in such events indicates the existence of
a causal link between them, but the mechanisms by which one eruption
can trigger another one remain largely a mystery. A particularly
beautiful example of a global sympathetic event was recently observed
by the Solar Dynamics Observatory (SDO) on 1 August 2010. It included
a small filament eruption and CME that was closely followed by the
eruptions of two large adjacent (twin) filaments, indicating that these
three eruptions were physically connected. A coronal potential field
extrapolation revealed that the twin filaments were located in the
lobes of a so-called pseudostreamer prior to their eruptions. Here we
present a 3D MHD simulation of the successive eruption of two magnetic
flux ropes in such a pseudostreamer configuration. The two eruptions are
triggered by the simulated eruption of a third flux rope in the vicinity
of the pseudostreamer. The simulation qualitatively reproduces the CME
and subsequent twin filament eruption on 1 August 2010 and suggests that
these events were indeed physically connected. Furthermore, it provides
a generic scenario for the frequently observed twin filament eruptions
in coronal pseudostreamers and suggests a mechanism by which such
eruptions can be triggered in the first place. Our results thus provide
an important step for a better understanding of sympathetic eruptions.
Title: How Geometric Factors Affect Coronal Loop Properties
Authors: Mok, Yung; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2011SPD....42.1817M
Altcode: 2011BAAS..43S.1817M
We studied over 200 closed field lines from a realistic FF field in
the neighborhood of a sunspot group. Each field line can be perceived
as a coronal loop with plasma density and temperature profiles when
appropriate heating and radiative cooling are applied. These field
lines have a variety of properties, including spatially varying
flux tube area and the geometric shape that determines gravity
projection. As a result, we see a large range of loop behaviors. Some
reach a static equilibrium. Some reach a steady state with a one-way
siphon flow. However, most of them do not reach a steady state. Their
temperature/density oscillate in time between a hot phase and a cool
phase periodically, with approximately 90 degrees phase difference
between temperature and density. By artificially modifying their
geometric properties and/or heating, we show that we can alter their
behaviors, for example, from an oscillating type to a steady-state
type. Our study underscores the importance of using the correct geometry
when using 1D simulation to model coronal loops.
Title: Magnetohydrodynamic Modeling of the Origin and Evolution of
Corotating Interaction Regions
Authors: Linker, Jon A.; Riley, P.; Lionello, R.; Mikic, Z.;
Stevens, M.
Bibcode: 2011SPD....42.1405L
Altcode: 2011BAAS..43S.1405L
Recurrent geomagnetic activity at Earth is closely associated with
corotating interaction regions (CIRs), which form when fast solar
streams catch up to and interact with slow solar wind. CIRs are most
often associated with the declining phase of the solar cycle. In the
classic picture of the formation of CIRs, fast solar wind streams
emanating from the polar coronal holes encounter and compress the slow
solar wind in the ecliptic because of the overall tilt of the Sun's
magnetic axis during this phase of the cycle. This picture fits well
with Ulysses observations near the end of solar cycle 22. In the most
recent solar minimum, recurrent fast solar wind streams at Earth were
frequently observed, but these streams were associated with isolated
equatorial coronal holes, rather than the extended polar coronal
holes of the previous minimum. This time period would seem ideal for
testing models of coronal and heliospheric structure, which were first
developed and tested against observations at the end of cycle 22. We
have developed MHD models of the corona and solar wind for Carrington
rotation 2060 (Aug. 14 - Sept. 10, 2007), when prominent fast solar
wind streams were present. We discovered that the model predictions
varied significantly depending on which solar observatory was used to
develop the boundary conditions, and that the models systematically
underestimated the strength of B in the solar wind. We discuss
our results thus far in identifying the underlying causes of these
discrepancies and some of the implications for providing routine models
of the corona and solar wind. Research supported by NASA and NSF.
Title: MHD Simulations of Coronal Plumes
Authors: Lionello, Roberto; Velli, M.; Linker, J. A.; Mikic, Z.
Bibcode: 2011SPD....42.1807L
Altcode: 2011BAAS..43S.1807L
The expansion of a coronal hole filled with a discrete number of
higher density coronal plumes is simulated using a time-dependent 2D
code. A solar wind model including an exponential coronal heating
function and a flux of Alfven waves propagating both inside and
outside the structures is taken as a basic state. Different plasma
plume profiles are obtained by using different scale heights for the
heating rates. Remote sensing and solar wind in situ observations are
used to constrain the parameter range of the study. Time dependence
due to plume ignition and disappearance is also discussed. Velocity
differences of the order of 50 km/s, such as those found in microstreams
in the high-speed solar wind, may be easily explained by slightly
different heat deposition profiles in different plumes. Statistical
pressure balance in the fast wind data may be masked by the large
variety of body and surface waves which the higher density filaments
may carry, so the absence of pressure balance in the microstreams
should not rule out their interpretation as the extension of coronal
plumes into interplanetary space. Mixing of plume-interplume material
via the Kelvin-Helmholtz instability seems to be possible, within the
parameter ranges of the models defined here, only at large distances
from the Sun, beyond 0.2-0.3 AU. Plasma and composition measurements
in the inner heliosphere, such as those which will become available
with Solar Orbiter and Solar Probe Plus, should therefore definitely
be able to identify plume remnants in the solar wind.
Title: Magnetic Topology of the Source Surface Potential Field on
1 August 2010
Authors: Titov, Viacheslav; Mikic, Z.; Torok, T.; Linker, J. A.
Bibcode: 2011SPD....42.2303T
Altcode: 2011BAAS..43S.2303T
A sequence of coronal mass ejections was recently observed by the Solar
Dynamics Observatory (SDO) on 1 August 2010. The events were closely
synchronized with one another, even though some of them occured at
rather different locations. Therefore, it is tempting to assume that
these events were causally linked with each other. In an attempt to
verify this assumption and identify a plausible reason of such a link,
we study the topological structure of the source surface potential
field that has been computed from the observed photospheric magnetic
field at the appropriate time period. For this purpose, we investigate
the respective magnetic connectivity in the obtained configuration by
calculating and analyzing the distributions of the so-called squashing
factor at the boundaries as well as at different cross-sections. This
allows us to get a comprehensive understanding of the underlying
structural skeleton of the magnetic cofiguration and identify the
robust topological features that likely establish the assumed causal
link in the indicated events. The obtained topological framework also
provides a solid guide for further numerical modeling and analysis of
the observational data of these eruptions.
Title: Magnetic Topology of Coronal Hole Linkages
Authors: Titov, V. S.; Mikić, Z.; Linker, J. A.; Lionello, R.;
Antiochos, S. K.
Bibcode: 2011ApJ...731..111T
Altcode: 2010arXiv1011.0009T
In recent work, Antiochos and coworkers argued that the boundary between
the open and closed field regions on the Sun can be extremely complex
with narrow corridors of open flux connecting seemingly disconnected
coronal holes from the main polar holes and that these corridors may be
the sources of the slow solar wind. We examine, in detail, the topology
of such magnetic configurations using an analytical source surface model
that allows for analysis of the field with arbitrary resolution. Our
analysis reveals three new important results. First, a coronal hole
boundary can join stably to the separatrix boundary of a parasitic
polarity region. Second, a single parasitic polarity region can produce
multiple null points in the corona and, more important, separator
lines connecting these points. It is known that such topologies are
extremely favorable for magnetic reconnection, because they allow
this process to occur over the entire length of the separators rather
than being confined to a small region around the nulls. Finally, the
coronal holes are not connected by an open-field corridor of finite
width, but instead are linked by a singular line that coincides with
the separatrix footprint of the parasitic polarity. We investigate
how the topological features described above evolve in response to
the motion of the parasitic polarity region. The implications of our
results for the sources of the slow solar wind and for coronal and
heliospheric observations are discussed.
Title: A Model for the Sources of the Slow Solar Wind
Authors: Antiochos, S. K.; Mikić, Z.; Titov, V. S.; Lionello, R.;
Linker, J. A.
Bibcode: 2011ApJ...731..112A
Altcode: 2011arXiv1102.3704A
Models for the origin of the slow solar wind must account for two
seemingly contradictory observations: the slow wind has the composition
of the closed-field corona, implying that it originates from the
continuous opening and closing of flux at the boundary between open
and closed field. On the other hand, the slow wind also has large
angular width, up to ~60°, suggesting that its source extends far
from the open-closed boundary. We propose a model that can explain
both observations. The key idea is that the source of the slow wind
at the Sun is a network of narrow (possibly singular) open-field
corridors that map to a web of separatrices and quasi-separatrix
layers in the heliosphere. We compute analytically the topology of an
open-field corridor and show that it produces a quasi-separatrix layer
in the heliosphere that extends to angles far from the heliospheric
current sheet. We then use an MHD code and MDI/SOHO observations of
the photospheric magnetic field to calculate numerically, with high
spatial resolution, the quasi-steady solar wind, and magnetic field
for a time period preceding the 2008 August 1 total solar eclipse. Our
numerical results imply that, at least for this time period, a web of
separatrices (which we term an S-web) forms with sufficient density
and extent in the heliosphere to account for the observed properties
of the slow wind. We discuss the implications of our S-web model for
the structure and dynamics of the corona and heliosphere and propose
further tests of the model.
Title: The Evolution of Open Magnetic Flux Driven by Photospheric
Dynamics
Authors: Linker, Jon A.; Lionello, Roberto; Mikić, Zoran; Titov,
Viacheslav S.; Antiochos, Spiro K.
Bibcode: 2011ApJ...731..110L
Altcode:
The coronal magnetic field is of paramount importance in solar and
heliospheric physics. Two profoundly different views of the coronal
magnetic field have emerged. In quasi-steady models, the predominant
source of open magnetic field is in coronal holes. In contrast, in the
interchange model, the open magnetic flux is conserved, and the coronal
magnetic field can only respond to the photospheric evolution via
interchange reconnection. In this view, the open magnetic flux diffuses
through the closed, streamer belt fields, and substantial open flux is
present in the streamer belt during solar minimum. However, Antiochos
and coworkers, in the form of a conjecture, argued that truly isolated
open flux cannot exist in a configuration with one heliospheric current
sheet—it will connect via narrow corridors to the polar coronal
hole of the same polarity. This contradicts the requirements of the
interchange model. We have performed an MHD simulation of the solar
corona up to 20 R sun to test both the interchange model
and the Antiochos conjecture. We use a synoptic map for Carrington
rotation 1913 as the boundary condition for the model, with two small
bipoles introduced into the region where a positive polarity extended
coronal hole forms. We introduce flows at the photospheric boundary
surface to see if open flux associated with the bipoles can be moved
into the closed-field region. Interchange reconnection does occur in
response to these motions. However, we find that the open magnetic
flux cannot be simply injected into closed-field regions—the flux
eventually closes down and disconnected flux is created. Flux either
opens or closes, as required, to maintain topologically distinct open-
and closed-field regions, with no indiscriminate mixing of the two. The
early evolution conforms to the Antiochos conjecture in that a narrow
corridor of open flux connects the portion of the coronal hole that
is nearly detached by one of the bipoles. In the later evolution,
a detached coronal hole forms, in apparent violation of the Antiochos
conjecture. Further investigation reveals that this detached coronal
hole is actually linked to the extended coronal hole by a separatrix
footprint on the photosphere of zero width. Therefore, the essential
idea of the conjecture is preserved, if we modify it to state that
coronal holes in the same polarity region are always linked, either
by finite width corridors or separatrix footprints. The implications
of these results for interchange reconnection and the sources of the
slow solar wind are briefly discussed.
Title: MHD Simulations of the Global Solar Corona and the Solar Wind
Authors: Lionello, Robert; Linker, Jon A.; Mikić, Zoran; Riley,
Pete; Titov, Viacheslav, S.
Bibcode: 2011sswh.book..101L
Altcode:
No abstract at ADS
Title: Characterization of the slow wind in the outer corona
Authors: Abbo, Lucia; Antonucci, Ester; Mikić, Zoran; Linker, Jon A.;
Riley, Pete; Lionello, Roberto
Bibcode: 2010AdSpR..46.1400A
Altcode: 2010arXiv1008.4452A
The study concerns the streamer belt observed at high spectral
resolution during the minimum of solar cycle 23 with the Ultraviolet
Coronagraph Spectrometer (UVCS) onboard SOHO. On the basis of
a spectroscopic analysis of the O VI doublet, the solar wind
plasma parameters are inferred in the extended corona. The analysis
accounts for the coronal magnetic topology, extrapolated through a 3D
magneto-hydrodynamic model, in order to define the streamer boundary
and to analyse the edges of coronal holes. The results of the analysis
allow an accurate identification of the source regions of the slow
coronal wind that are confirmed to be along the streamer boundary in
the open magnetic field region.
Title: The Wide Field Imager for Solar PRobe (WISPR)
Authors: Plunkett, S. P.; Howard, R. A.; Vourlidas, A.; Korendyke,
C. M.; Socker, D. G.; Morrill, J. S.; Sheeley, N. R.; Linton, M.;
Liewer, P. C.; de Jong, E. M.; Mikic, Z.
Bibcode: 2010AGUFMSH11B1622P
Altcode:
The Wide Field Imager for Solar PRobe (WISPR) will image the
Thomson-scattered light from the coronal plasma in the inner corona,
with unprecedented spatial resolution, cadence, and sensitivity. WISPR
follows on the SECCHI Heliospheric Imager (HI) aboard the STEREO
mission, and addresses all four key objectives in the Solar Probe Plus:
Report of the STDT (2008): (1) Determine the structure and dynamics of
the magnetic fields at the sources of the fast and slow solar wind, (2)
Trace the flow of energy that heats the solar corona and accelerates
the SW, (3) explore the mechanisms that accelerate and transport
energetic particles, (4) explore dusty plasma phenomena and their
influence on the solar wind and energetic particle formation. Situated
in the ram direction of the Solar Probe Plus (SPP) spacecraft (S/C),
WISPR will have the unique ability to image the coronal structures when
they are close to the Sun, as they approach, and as they pass over the
spacecraft. As a remote sensor, WISPR will connect the structures close
to the Sun to the spacecraft and provide important spatial and temporal
information; measuring, for example, the properties of the structures
generating the shocks and SEPs that will be measured in a few minutes at
the S/C. Since the S/C is embedded in the corona, WISPR and the in situ
instruments will measure for the first time the same plasma. Also as
the SPP transits through the corona, the rapidly-varying viewpoint and
high spatial resolution of WISPR will enable tomographic imaging of the
corona, and lead to higher fidelity and finer scale 3D reconstructions
than are possible with the STEREO mission or single-view rotational
tomography. The wide field of view will include at times other inner
heliospheric probes (e.g. Solar Orbiter) and will image the outflowing
wind that is impinging on that probe. In addition to this standard
imaging mode, WISPR opens a new capability for imaging instruments,
the measurement of pressure turbulence by employing a high cadence mode
(~1 sec) to image a small region in the corona. For the first time,
the slopes of the power spectral density from images can be compared
directly to the density and magnetic field fluctuations seen in situ as
a function of coronal spatial structure and heliocentric distance. In
addition, the 1 sec cadence can be generated anywhere within the WISPR
field, enabling the study of the transition of the solar wind injected
at the tops of the helmet streamers to inertial dissipation scales.
Title: Comparing an MHD Model of the Corona During the July 11,
2010 Total Solar Eclipse with Observations (Invited)
Authors: Mikic, Z.; Linker, J. A.; Lionello, R.; Riley, P.; Titov,
V. S.
Bibcode: 2010AGUFMSH42A..09M
Altcode:
Total solar eclipses offer a unique opportunity to study the white
light and emission coronae at high resolution. Magnetohydrodynamic
(MHD) models have been used to predict the structure of the corona
prior to eclipses, using measurements of photospheric magnetic fields
on the Sun. In particular, such an MHD model was used to predict the
structure of the corona for the July 11, 2010 total solar eclipse,
using SOHO/MDI photospheric magnetic field data. We will compare
observed images of the total solar eclipse with features from the MHD
model, including magnetic field line traces and simulated polarization
brightness images. We will also compare images of simulated emission in
EUV and X-rays with observations from SOHO/EIT, Hinode/XRT, STEREO/EUVI,
and SDO/AIA. Such comparisons of observed emission with predictions
from global coronal MHD models provide a very sensitive constraint
on coronal heating models. Research supported by NASA's Heliophysics
Theory and Living With a Star Programs, and NSF/CISM.
Title: Thermal Nonequilibrium Revisited: a Heating Model for
Coronal Loops
Authors: Lionello, R.; Winebarger, A. R.; Linker, J. A.; Mikic, Z.;
Mok, Y.
Bibcode: 2010AGUFMSH31C1811L
Altcode:
The mechanism responsible for heating coronal loops is still a matter
of debate. To explain loop observations, steady uniform heating,
impulsive heating events of variable duration, and nanoflare heating
of elemental strands have been proposed. Thermal nonequilibrium was
discarded by Klimchuk et al. (2010) on the base of their 1D simulations,
as incapable of reproducing observational characteristics of loops. We
here revisit the viability of thermal nonequilibrium to explain the
physics of coronal loops, by comparing the results of 3D simulations
with observed properties in EUV and soft X-rays.
Title: Structure and Dynamics of the Erupting Magnetic Flux in the
May 12 1997 CME Event
Authors: Titov, V. S.; Mikic, Z.; Linker, J. A.; Lionello, R.
Bibcode: 2010AGUFMSH51C1687T
Altcode:
The identification of erupting magnetic fluxes in solar CMEs is a big
challenge from both computational and theoretical points of view. We
have attacked this problem by studying the May 12 1997 CME event
with the help of two powerful tools: (1) Our numerical MHD model of
erupting magnetic configurations; and (2) Our generalized field line
mapping technique for analyzing their magnetic structure. This approach
allows us to identify the building blocks of such configurations by
computing all their separatrix and quasi-separatrix surfaces that serve
as interfaces between such blocks. The latter include, in particular,
a flare arcade and erupting magnetic flux rope, which we relate to
the observed flare ribbons and EUV dimmings of the event. On the
basis of this analysis, we have also estimated the magnetic fluxes
associated with these blocks at several moments in time. This provides
a solid basis for a very detailed comparison of our MHD model with
observational data of this eruption. Such a comparison helps us to
verify our model and understand the physical processes and observed
peculiarities of this event in conjunction with the dynamics of the
underlying magnetic structure.
Title: A Parametric Study to Constrain Empirically-based Models of
the Ambient Solar Wind
Authors: Riley, P.; Linker, J. A.; Mikic, Z.
Bibcode: 2010AGUFMSH31B1799R
Altcode:
Current empirically-based models, driven by various features of
the coronal magnetic field often perform relatively well, (1)
in the absence of obvious transient phenomena, and (2) when the
ambient solar configuration remains relatively stable over several
or more rotations. However, even under these conditions, the models
can sometimes fail dramatically. Currently, there are three primary
techniques for predicting, in particular, solar wind speed at 1 AU
based on synoptic maps of the photospheric magnetic field. The original
Wang-Sheeley (WS) model uses an observed negative correlation between
solar wind speed and the expansion factor of the solar magnetic
field. The Predictive Science “Distance from the Coronal Hole
Boundary” (DCHB) model specifies speed in the photosphere based on
the perpendicular distance from the coronal hole boundary and maps
this speed out to 30 solar radii. And finally, the Wang-Sheeley-Arge
(WSA) model combines the WS and DCHB prescriptions. In this study, we
compare these three approaches for a set of carefully chosen Carrington
rotations. For each, we ran a suite of solutions using a range of
input parameters. We also generated solutions driven by synoptic
magnetograms from different observatories, since we have found that
they can significantly affect the resultant solutions. To directly
compare the model solutions with 1 AU in situ measurements at ACE,
Wind, STEREO A and B, and assess the potential impact of modeling stream
evolution, we used two global heliospheric models (Enlil and MAS). We
also employed an alternative and potentially more revealing approach
of dynamically mapping the in situ measurements back to a reference
surface at 30 solar radii and comparing them with the model maps.
Title: Magnetic Maps and Coronal/Solar Wind Modeling: Practices and
Pitfalls (Invited)
Authors: Linker, J. A.; Mikic, Z.; Riley, P.; Lionello, R.; Titov,
V. S.
Bibcode: 2010AGUFMSH41D..03L
Altcode:
The ambient solar corona and solar wind play a crucial role in solar
and heliospheric physics. The Sun's magnetic field is an essential
ingredient of any predictive model of the solar wind. It defines
the structure of the heliosphere, including the position of the
heliospheric current sheet and the regions of fast and slow solar
wind. The geoeffectiveness of CMEs is influenced in part by their
interaction with the ambient magnetic field, and the field determines
where SEPs propagate. To model the global magnetic field of the solar
corona, maps of the magnetic field over the entire solar surface
must be supplied as boundary conditions. In this talk, we demonstrate
how common errors and uncertainties in the field measurements can at
times strongly influence the solutions, and we discuss some of the
key challenges to improving magnetic maps. Research supported by CISM
(NSF), the LWS Strategic Capabilities Program (NASA, NSF, and AFOSR),
and Heliophysics Theory Program (NASA).
Title: Uncovering Mechanisms of Coronal Magnetism via Advanced 3D
Modeling of Flares and Active Regions
Authors: Fleishman, Gregory; Gary, Dale; Nita, Gelu; Alexander,
David; Aschwanden, Markus; Bastian, Tim; Hudson, Hugh; Hurford,
Gordon; Kontar, Eduard; Longcope, Dana; Mikic, Zoran; DeRosa, Marc;
Ryan, James; White, Stephen
Bibcode: 2010arXiv1011.2800F
Altcode:
The coming decade will see the routine use of solar data of
unprecedented spatial and spectral resolution, time cadence, and
completeness. To capitalize on the new (or soon to be available)
facilities such as SDO, ATST and FASR, and the challenges they present
in the visualization and synthesis of multi-wavelength datasets,
we propose that realistic, sophisticated, 3D active region and flare
modeling is timely and critical, and will be a forefront of coronal
studies over the coming decade. To make such modeling a reality, a
broad, concerted effort is needed to capture the wealth of information
resulting from the data, develop a synergistic modeling effort, and
generate the necessary visualization, interpretation and model-data
comparison tools to accurately extract the key physics.
Title: Interpretation of the cross-correlation function of ACE and
STEREO solar wind velocities using a global MHD Model
Authors: Riley, Pete; Luhmann, J.; Opitz, A.; Linker, J. A.; Mikic, Z.
Bibcode: 2010JGRA..11511104R
Altcode:
Measurements from the ACE and STEREO A and B spacecraft are allowing
an unprecedented view of the structure of the three-dimensional
heliosphere. One aspect of this is the degree to which the measurements
at one spacecraft correlate with those at the other. We have computed
the cross-correlation functions (CCFs) for all three combinations of
ACE and STEREO A and B in situ observations of the bulk solar wind
velocity as the spacecraft moved progressively farther away from
one another. Our results confirm previous studies that the phase lag
between the signals becomes linearly larger with time. However, we
have identified two intervals where this appears to break down. During
these "lulls," the CCF reveals a phase lag considerably less than
that which would be predicted based only on the angular separation
of the spacecraft. We modeled the entire STEREO time period using a
global MHD model to investigate the cause for these "lulls." We find
that a combination of time-dependent evolution of the streams as well
as spatial inhomogeneities, due to the latitudinal separation of the
spacecraft, are sufficient to explain them.
Title: Global MHD Modeling of the Solar Corona and Inner Heliosphere
for the Whole Heliosphere Interval
Authors: Riley, Pete; Linker, Jon A.; Mikic, Zoran
Bibcode: 2010HiA....15..491R
Altcode:
With the goal of understanding the three-dimensional structure of the
solar corona and inner heliosphere during the “Whole Heliosphere
Interval” (WHI), we have developed a global MHD solution for
Carrington rotation (CR) 2068. Our model, which includes energy
transport processes, such as coronal heating, conduction of heat
parallel to the magnetic field, radiative losses, and the effects
of Alfvén waves, is capable of producing significantly better
estimates of the plasma temperature and density in the corona than
have been possible in the past. With such a model, we can compute
emission in extreme ultraviolet (EUV) and X-ray wavelengths, as well as
scattering in polarized white light. Additionally, from our heliospheric
solutions, we can deduce magnetic field and plasma parameters along
specific spacecraft trajectories. We have made detailed comparisons
of both remote solar and in situ observations with the model results,
allowing us to: (1) Connect these disparate sets of observations;
(2) Infer the global structure of the inner heliosphere; and (3)
Provide support for (or against) assumptions in the MHD model, such
as the empirically-based coronal heating profiles.
Title: Current Sheet Energetics, Flare Emissions, and Energy Partition
in a Simulated Solar Eruption
Authors: Reeves, Katharine K.; Linker, Jon A.; Mikić, Zoran; Forbes,
Terry G.
Bibcode: 2010ApJ...721.1547R
Altcode:
We investigate coronal energy flow during a simulated coronal mass
ejection (CME). We model the CME in the context of the global corona
using a 2.5D numerical MHD code in spherical coordinates that includes
coronal heating, thermal conduction, and radiative cooling in the energy
equation. The simulation domain extends from 1 to 20 Rs
. To our knowledge, this is the first attempt to apply detailed
energy diagnostics in a flare/CME simulation when these important
terms are considered in the context of the MHD equations. We find
that the energy conservation properties of the code are quite good,
conserving energy to within 4% for the entire simulation (more than 6
days of real time). We examine the energy release in the current sheet
as the eruption takes place, and find, as expected, that the Poynting
flux is the dominant carrier of energy into the current sheet. However,
there is a significant flow of energy out of the sides of the current
sheet into the upstream region due to thermal conduction along field
lines and viscous drag. This energy outflow is spatially partitioned
into three separate components, namely, the energy flux flowing out
the sides of the current sheet, the energy flowing out the lower tip
of the current sheet, and the energy flowing out the upper tip of the
current sheet. The energy flow through the lower tip of the current
sheet is the energy available for heating of the flare loops. We
examine the simulated flare emissions and energetics due to the
modeled CME and find reasonable agreement with flare loop morphologies
and energy partitioning in observed solar eruptions. The simulation
also provides an explanation for coronal dimming during eruptions and
predicts that the structures surrounding the current sheet are visible
in X-ray observations.
Title: From the Sun to the Earth: The 13 May 2005 Coronal Mass
Ejection
Authors: Bisi, M. M.; Breen, A. R.; Jackson, B. V.; Fallows, R. A.;
Walsh, A. P.; Mikić, Z.; Riley, P.; Owen, C. J.; Gonzalez-Esparza,
A.; Aguilar-Rodriguez, E.; Morgan, H.; Jensen, E. A.; Wood, A. G.;
Owens, M. J.; Tokumaru, M.; Manoharan, P. K.; Chashei, I. V.; Giunta,
A. S.; Linker, J. A.; Shishov, V. I.; Tyul'bashev, S. A.; Agalya, G.;
Glubokova, S. K.; Hamilton, M. S.; Fujiki, K.; Hick, P. P.; Clover,
J. M.; Pintér, B.
Bibcode: 2010SoPh..265...49B
Altcode: 2010SoPh..tmp..136B
We report the results of a multi-instrument, multi-technique,
coordinated study of the solar eruptive event of 13 May 2005. We
discuss the resultant Earth-directed (halo) coronal mass ejection
(CME), and the effects on the terrestrial space environment and
upper Earth atmosphere. The interplanetary CME (ICME) impacted the
Earth's magnetosphere and caused the most-intense geomagnetic storm
of 2005 with a Disturbed Storm Time (Dst) index reaching −263 nT
at its peak. The terrestrial environment responded to the storm on
a global scale. We have combined observations and measurements from
coronal and interplanetary remote-sensing instruments, interplanetary
and near-Earth in-situ measurements, remote-sensing observations and
in-situ measurements of the terrestrial magnetosphere and ionosphere,
along with coronal and heliospheric modelling. These analyses are used
to trace the origin, development, propagation, terrestrial impact, and
subsequent consequences of this event to obtain the most comprehensive
view of a geo-effective solar eruption to date. This particular event
is also part of a NASA-sponsored Living With a Star (LWS) study and
an on-going US NSF-sponsored Solar, Heliospheric, and INterplanetary
Environment (SHINE) community investigation.
Title: A Global MHD Model of the December 12, 2008 CME
Authors: Riley, Pete; Mikic, Zoran; Linker, Jon A.; Plunkett, Simon
Bibcode: 2010shin.confE.146R
Altcode:
In this study, we present preliminary modeling results for the
December 12th, 2008 CME (observed December 16-17 in situ). The
event was likely associated with a filament eruption at N50-W15,
which started at 04:00UT. H-alpha observations show the presence of
a substantial filament prior to eruption and STEREO A and B captured
it in the form of an erupting prominence off the east and west limbs,
respectively. Using an MDI synoptic map of Carrington rotation 2077, we
constructed an ambient global MHD solution of the corona prior to the
eruption. Since the observations suggest a polar crown filament (PCF)
event, a likely mechanism for its eruption involves the emergence of
magnetic field along the neutral line producing the prominence, followed
by converging flow toward that neutral line. H-alpha observations
superimposed on the global photospheric neutral line, derived from
the MDI magnetogram, allow us to identify the likely region to apply
this flux emergence and converging flow. We present results from the
model and compare with both solar and in situ observations.
Title: The 13-15 May 2005 CME/ICME/MC: A Comprehensive Study from
the Sun to the Earth
Authors: Bisi, Mario Mark; Breen, A. R.; Jackson, B. V.;
Fallows, R. A.; Walsh, A. P.; Owens, M. J.; Riley, P.; Mikić,
Z.; Gonzalez-Esparza, A.; Aguilar-Rodriguez, E.; Morgan, H.; Wood,
A. G.; Jensen, E. A.; Tokumaru, M.; Manoharan, P. K.; Chashei, I. V.;
Giunta, A. S.; Owen, C. J.; Fujiki, K.; Linker, J. A.; Shishov, V. I.;
Tyul'bashev, S. A.; Agalya, G.; Glubokova, S. K.
Bibcode: 2010shin.confE.126B
Altcode:
Here, we present a brief overview of the results of a multi-technique,
multi-instrument, co-ordinated study of the solar-eruptive and
Earth-effective event(s) of 13-15 May 2005. We look at the resulting
Earth-directed (halo) coronal mass ejection (CME), the interplanetary
counterpart (ICME), and briefly, the flux-rope (Magnetic Cloud -
MC) effects on the terrestrial space environment and upper Earth
atmosphere. We have combined observations and measurements from
coronal and interplanetary remote-sensing instruments, interplanetary
and near-Earth in-situ measurements, remote-sensing observations and
in-situ measurements of the terrestrial magnetosphere and ionosphere,
as well as the use of coronal and heliospheric modelling. These analyses
are subsequently used to trace the origin, development, propagation,
terrestrial impact, and consequences of this event to obtain the
most-comprehensive view (to our knowledge) of an Earth-effective solar
eruption to date. Full details of the study of this event can be found
in a comprehensive paper by Bisi et al., Solar Physics, Topical Issue
(TI) on Remote Sensing of the Inner Heliosphere, 2010, when the TI is
published in August/September 2010.
Title: Coronal Mass Ejection Initiation: On the Nature of the Flux
Cancellation Model
Authors: Amari, T.; Aly, J. -J.; Mikic, Z.; Linker, J.
Bibcode: 2010ApJ...717L..26A
Altcode: 2010arXiv1005.4669A
We consider a three-dimensional bipolar force-free magnetic field with a
nonzero magnetic helicity, occupying a half-space, and study the problem
of its evolution driven by an imposed photospheric flux decrease. For
this specific setting of the Flux Cancellation Model describing
coronal mass ejections occurring in active regions, we address the
issues of the physical meaning of flux decrease, of the influence on
field evolution of the size of the domain over which this decrease is
imposed, and of the existence of an energetic criterion characterizing
the possible onset of disruption of the configuration. We show that
(1) the imposed flux disappearance can be interpreted in terms of
transport of positive and negative fluxes toward the inversion line,
where they get annihilated. (2) For the particular case actually
computed, in which the initial state is quite sheared, the formation
of a twisted flux rope and the subsequent global disruption of the
configuration are obtained when the flux has decreased by only a
modest amount over a limited part of the whole active region. (3)
The disruption is produced when the magnetic energy becomes of the
order of the decreasing energy of a semi-open field, and then before
reaching the energy of the associated fully open field. This suggests
that the mechanism leading to the disruption is nonequilibrium as in
the case where flux is imposed to decrease over the whole region.
Title: Magnetic Topology of Coronal Hole Linkages
Authors: Titov, Viacheslav S.; Mikic, Zoran; Linker, Jon A.; Lionello,
Roberto; Antiochos, Spiro
Bibcode: 2010shin.confE.120T
Altcode:
In recent work, Antiochos et al. (2007) argued that the boundary between
the open and closed field regions on the Sun can be extremely complex
with narrow corridors of open flux connecting seemingly disconnected
coronal holes from the main polar holes, and that these corridors
may be the sources of the slow solar wind. We examine, in detail, the
topology of such magnetic configurations using an analytical source
surface model that allows for analysis of the field with arbitrary
resolution. Our analysis reveals three important new results: First,
a coronal hole boundary can include the separatrix boundary of a
parasitic polarity region. Second, a single parasitic polarity region
can produce multiple null points in the corona and, more important,
separator lines connecting these points. Such topologies are extremely
favorable for magnetic reconnection, because it can now occur over
the entire length of the separators rather than being confined to
a small region around the nulls. Finally, the coronal holes are not
connected by an open-field corridor of finite width, but instead are
linked by a singular line that coincides with the separatrix footprint
of the parasitic polarity. We investigate how the topological features
described above evolve in response to motion of the parasitic polarity
region. The implications of our results for the sources of the slow
wind and for coronal and heliospheric observations are discussed.
Title: Modeling of the Ambient Solar Corona and Solar Wind: How
Magnetic Maps can Make or Break your Solution
Authors: Linker, Jon A.; Mikic, Zoran; Riley, Pete; Lionello, Roberto;
Titov, Viacheslav
Bibcode: 2010shin.confE..55L
Altcode:
The ambient solar corona and solar wind play a crucial role in solar
and heliospheric physics. The Sun's magnetic field is an essential
ingredient of any predictive model of the solar wind. It defines
the structure of the heliosphere, including the position of the
heliospheric current sheet and the regions of fast and slow solar
wind. The geoeffectiveness of CMEs is influenced in part by their
interaction with the ambient magnetic field, and the field determines
where SEPs propagate. To model the global magnetic field of the solar
corona, maps of the magnetic field over the entire solar surface
must be supplied as boundary conditions. In this talk, we demonstrate
how common errors and uncertainties in the field measurements can at
times strongly influence the solutions, and we discuss some of the
key challenges to improving magnetic maps.
Title: Computing the DEM out of MHD Simulations and Comparing It
with Observations
Authors: Lionello, Roberto; Linker, Jon A.; Mikić, Zoran; Mok, Yung
Bibcode: 2010shin.confE..13L
Altcode:
We show how to compute differential emission measure, or DEM, out of the
temperature and density distributions obtained from 3D MHD simulations
of the solar corona. Our 3D MHD model has thermal conduction, radiative
losses, and coronal heating terms in the energy equation and has been
used extensively to model realistic configurations of the corona and
active regions. Now we can now compare with the observations not only
the emission in EUV and X ray bands but also the DEM. Comparisons with
July 2010 eclipse will be presented.
Title: MHD Simulations of the May 13, 2005 CME Event
Authors: Mikic, Zoran; Linker, Jon A.; Lionello, Roberto; Riley,
Pete; Titov, Viacheslav
Bibcode: 2010shin.confE.125M
Altcode:
We will describe progress on our 3D MHD simulations of the May 13,
2005 CME Event.
Title: On the relationship between coronal heating, magnetic flux,
and the density of the solar wind
Authors: Riley, Pete; Mikic, Z.; Lionello, R.; Linker, J. A.;
Schwadron, N. A.; McComas, D. J.
Bibcode: 2010JGRA..115.6104R
Altcode: 2010JGRA..11506104R
The stark differences between the current solar minimum and the
previous one offer a unique opportunity to develop new constraints on
mechanisms for heating and acceleration of the solar wind. We have
used a combination of numerical simulations and analysis of remote
solar and in situ observations to infer that the coronal heating rate,
H, scales with the average magnetic field strength within a coronal
hole, Bch. This was accomplished in three steps. First,
we analyzed Ulysses measurements made during its first and third
orbit southern and northern polar passes (i.e., during near-solar
minimum conditions) to deduce a linear relationship between proton
number density (np) and radial magnetic field strength
(Br) in the high-speed quiescent solar wind, consistent with
the results of McComas et al. (2008) and Ebert et al. (2009). Second,
we used Wilcox Solar Observatory measurements of the photospheric
magnetic field to show that the magnetic field strength within coronal
holes (Bch) is approximately correlated with the strength
of the interplanetary field at the location of Ulysses. Third, we
used hydrodynamic simulations to show that np in the solar
wind scales linearly with H. Taken together, these results imply the
chain: H $\propto$ np $\propto$ Br $\propto$
Bch. We also explored ideas that the correlation between
np and Br could have resulted from interplanetary
processes, or from the superradial expansion of the coronal magnetic
field close to the Sun, but find that neither possibility can produce
the observed relationship. The derived heating relationship is
consistent with (1) empirical heating laws derived for closed-field
line regions and (2) theoretical models aimed at understanding both
the heating and acceleration of the solar wind.
Title: Structural Analysis of the Coronal Magnetic Field
Authors: Titov, Viacheslav; Mikic, Z.; Linker, J.; Lionello, R.
Bibcode: 2010AAS...21640607T
Altcode: 2010BAAS...41..880T
Recent developments in the analysis of the 3D magnetic field structure
have given us new insights into the properties of coronal mass ejections
and the solar wind. Maps of the flux-tube squashing factor Q make
it possible to identify separatrix and quasi-separatrix surfaces
that delimit "building blocks" of the magnetic configuration. When combined with high resolution MHD models of active regions,
this technique gives us a powerful way to relate observed features
with the underlying properties of the magnetic field, and to analyze
the nature and rate of 3D magnetic reconnection. On a global scale,
in particular, our method reveals the intricate structure of coronal
holes and localizes a possible source of the slow solar wind. We will
show how we have used this method to understand the observed properties
of flare ribbons, flare arcades, and EUV and X-ray dimming regions in
connection with the properties of the underlying erupting flux rope. Work supported by NASA and the Center for Integrated Space Weather
Modeling (an NSF Science and Technology Center).
Title: Interpreting Small-Scale Structure from High Resolution Global
MHD Simulations
Authors: Mikic, Zoran; Titov, V. S.; Linker, J. A.; Lionello, R.;
Riley, P.; Antiochos, S.
Bibcode: 2010AAS...21640503M
Altcode: 2010BAAS...41..889M
High resolution 3D MHD simulations of the solar corona are beginning
to reveal how small-scale structures in the magnetic field interact
with the global structure of the corona and solar wind. In particular,
it has become evident that the detailed characteristics of coronal
holes, especially their equatorial extensions, may be related to the
source of the slow solar wind. Using structural analysis based on the
squashing factor Q (Titov et al. 2002, 2008; Titov 2007) we show how
small-scale structure in the magnetic field is related to the structure
of the streamer belt. These results have led to a new interpretation
of the source of the slow solar wind. Research supported by NASA's
Heliospheric Theory and Living With a Star Programs, and NSF/CISM.
Title: Comparison of 3D and 1D Coronal Loop Simulations and Their
Implied Emissions
Authors: Mok, Yung; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2010AAS...21630003M
Altcode:
The thermal structure of coronal loops have been studied by numerous 1D
simulations and a few 3D simulations. Each method has its advantages
and disadvantages. 1D allows high resolution with a large number of
grid points to resolve the steep transition region without consuming
excessive computer time. However, it provides no information on
the internal structure over the loop's cross section, and lacks
3D perspective when compared with observations. 3D simulations,
however, are often handicapped by limited spatial resolution. We have
demonstrated the formation of coronal loops first by 3D simulation
over an active region. The loops appear to be thin with uniform cross
section in EUV as observed, despite the cross section expansion of
the underlying flux tube. Then, we extracted the field line that
threads through the loop, and did a 1D high-resolution simulation. In
this approach, we used the strength of each method to validate the
results. More importantly, the computed EUV emissions from the loops
must be compared with observations. In this case, only 3D simulations
can provide the necessary line-of-sight-integration effects and allow
us to view the loops from different perspectives. Our presentation
will show a realistic 3D view of the loops over an active region in
EUV. Work supported by Heliospheric Physics Theory Program of NASA
Title: An MHD Model with Wave Turbulence Driven Heating and Solar
Wind Acceleration
Authors: Lionello, Roberto; Linker, J. A.; Mikic, Z.; Riley, P.;
Velli, M.
Bibcode: 2010AAS...21630301L
Altcode:
The mechanisms responsible for heating the Sun's corona and accelerating
the solar wind are still being actively investigated. However, it
is largely accepted that photospheric motions provide the energy
source and that the magnetic field must play a key role in the
process. Three-dimensional MHD models have traditionally used an
empirical prescription for coronal heating (e.g., Lionello et al. 2009),
together with WKB Alfven wave acceleration of the solar wind. Recently,
attention has been focused on wave turbulence driven models (e.g.,
Cranmer et al. 2007; Cranmer 2010) in which the heating and solar
wind acceleration by Alfven waves are included self-consistently. We
will demonstrate the initial implementation of this idea in an MHD
model based on turbulent cascade heating in the closed-field regions
(Rappazzo et al. 2007, 2008), and Alfven wave turbulent dissipation
in open field regions (Verdini & Velli 2007, 2010).
Title: A Model for the Sources of the Slow Solar Wind
Authors: Antiochos, Spiro K.; Mikic, Z.; Lionello, R.; Titov, V.;
Linker, J.
Bibcode: 2010AAS...21640521A
Altcode: 2010BAAS...41..892A
Models for the origin of the slow solar wind must account for
two seemingly contradictory observations: The slow wind has the
composition of the closed-field corona, implying that it originates at
the open-closed field boundary layer, but it also has large angular
width, up to 40 degrees. We propose a model that can explain both
observations. The key idea is that the source of the slow wind at the
Sun is a network of narrow (possibly singular) open-field corridors
that map to a web of separatrices and quasi-separatrix layers in
the heliosphere. We calculate with high numerical resolution, the
quasi-steady solar wind and magnetic field for a Carrington rotation
centered about the August 1, 2008 total solar eclipse. Our numerical
results demonstrate that, at least for this time period, a web of
separatrices (S-web) forms with sufficient density and extent in
the heliosphere to account for the observed properties of the slow
wind. We discuss the implications of our S-web model for the structure
and dynamics of the corona and heliosphere, and propose further tests
of the model. This work was supported, in part, by the NASA HTP,
TR&T and SR&T programs.
Title: CORHEL MHD Modeling in Support of Solar Dynamics Observatory
Authors: Linker, Jon A.; Riley, P.; Mikic, Z.; Lionello, R.; Titov,
V.; Wijaya, J.
Bibcode: 2010AAS...21640217L
Altcode: 2010BAAS...41..876L
CORHEL - for Corona-Heliosphere - is a coupled set of models and tools
for quantitatively modeling the ambient solar corona and solar wind
in various approximations. The coronal MHD code MAS in CORHEL has been
used to produce routine polytropic solutions for all of the Carrington
rotations during the STEREO mission (available at www.predsci.com). The
MAS code can also be used to produce solutions that include energy
transport (radiative losses, anisotropic thermal conduction, and coronal
heating) in the transition region and solar corona. This more accurate
representation of energy flow allows us to compute simulated EUV and
X-ray emission and compare directly with observations. We refer to
this as the thermodynamic MHD model. In this paper, we describe the
production of thermodynamic MHD solutions as part of CORHEL. When
sufficiently calibrated data are available, the solutions will use
magnetic maps derived from HMI magnetograms. These solutions will be
made routinely available in support of the Solar Dynamics Observatory
(SDO) mission, and will allow comparison with emission observations
from AIA when emission kernels become available. Work supported
by the LWS Strategic Capabilities Program (NASA, NSF, and AFOSR), CISM
(NSF), HTP (NASA) and the HMI team.
Title: Low-Latitude Coronal Holes at the Minimum of the 23rd Solar
Cycle
Authors: Abramenko, Valentyna; Yurchyshyn, Vasyl; Linker, Jon; Mikić,
Zoran; Luhmann, Janet; Lee, Christina O.
Bibcode: 2010ApJ...712..813A
Altcode: 2010arXiv1002.1685A
Low- and mid-latitude coronal holes (CHs) observed on the Sun during
the current solar activity minimum (from 2006 September 21, Carrington
rotation (CR) 2048, to 2009 June 26, CR 2084) were analyzed using Solar
and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope and
STEREO-A SECCHI EUVI data. From both the observations and Potential
Field Source Surface modeling, we find that the area occupied by
CHs inside a belt of ±40° around the solar equator is larger in
the current 2007 solar minimum relative to the similar phase of the
previous 1996 solar minimum. The enhanced CH area is related to a
recurrent appearance of five persistent CHs, which survived during 7-27
solar rotations. Three of the CHs are of positive magnetic polarity
and two are negative. The most long-lived CH was being formed during
2 days and existed for 27 rotations. This CH was associated with fast
solar wind at 1 AU of approximately 620 ± 40 km s-1. The
three-dimensional magnetohydrodynamic modeling for this time period
shows an open field structure above this CH. We conclude that the global
magnetic field of the Sun possessed a multi-pole structure during
this time period. Calculation of the harmonic power spectrum of the
solar magnetic field demonstrates a greater prevalence of multi-pole
components over the dipole component in the 2007 solar minimum compared
to the 1996 solar minimum. The unusual large separation between the
dipole and multi-pole components is due to the very low magnitude
of the dipole component, which is three times lower than that in the
previous 1996 solar minimum.
Title: Comparing eclipse observations of the 2008 August 1 solar
corona with an MHD model prediction
Authors: Rušin, V.; Druckmüller, M.; Aniol, P.; Minarovjech, M.;
Saniga, M.; Mikić, Z.; Linker, J. A.; Lionello, R.; Riley, P.; Titov,
V. S.
Bibcode: 2010A&A...513A..45R
Altcode:
Context. The structure of the white-light and emission solar coronas
and their MHD modelling are the context of our work.
Aims:
A comparison is made between the structure of the solar corona as
observed during the 2008 August 1 total eclipse from Mongolia and
that predicted by an MHD model.
Methods: The model has an
improved energy formulation, including the effect of coronal heating,
conduction of heat parallel to the magnetic field, radiative losses,
and acceleration by Alfvén waves.
Results: The white-light
corona, which was visible up to 20 solar radii, was of an intermediate
type with well-pronounced helmet streamers situated above a chain of
prominences at position angles of 48, 130, 241, and 322 degrees. Two
polar coronal holes, filled with a plethora of thin polar plumes,
were observed. High-quality pictures of the green (530.3 nm, Fe XIV)
corona were obtained with the help of two narrow-passband filters
(centered at the line itself and the vicinity of 529.1 nm background),
with a FWHM of 0.15 nm.
Conclusions: The large-scale shape of
both the white-light and green corona was found to agree well with that
predicted by the model. In this paper we describe the morphological
properties of the observed corona, and how it compares with that
predicted by the model. A more detailed analysis of the quantitative
properties of the corona will be addressed in a future publication.
Title: CORHEL: A Modeling Suite for Describing the Solar Corona and
Inner Heliosphere
Authors: Linker, J. A.; Riley, P.; Mikic, Z.; Lionello, R.; Titov,
V. S.; Odstrcil, D.
Bibcode: 2009AGUFMSA43A1612L
Altcode:
The Sun and its activity is the ultimate driver of space weather at
Earth. The ambient solar corona and solar wind plays a key role in
transmitting solar activity to the Earth's space environment. Coronal
mass ejections (CMEs) propagate and interact with the ambient solar
wind; their geoeffectiveness is affected by this interaction. The
connection of the ambient interplanetary magnetic field to CME-related
shocks and impulsive solar flares determines where solar energetic
particles propagate. The partitioning of the ambient solar wind
into fast and slow streams is responsible for recurrent geomagnetic
activity. CORHEL - for Corona-Heliosphere - is a coupled set of
models and tools for quantitatively modeling the ambient solar
corona and solar wind in various approximations. It includes two
coronal models (the MHD model MAS, and a new implementation of the
empirical WSA model) and two heliospheric models (the MHD model Enlil
and a heliospheric version of MAS). The primary data input to CORHEL
consists of maps of the Sun's photospheric magnetic field derived from
magnetograms; data from 6 different observatories can be downloaded
and processed. CORHEL solutions are available to the community at the
CCMC (http://ccmc.gsfc.nasa.gov) and the Predictive Science web sites
(http://www.predsci.com). In this talk we describe progress in CORHEL
development and the challenges that remain for transitioning modeling of
the ambient corona and solar wind to an operational environment. CORHEL
is supported by CISM and the LWS Strategic Capabilities Program (NASA,
NSF, and AFOSR).
Title: Comparing High-Resolution MHD Simulations of the Present and
Preceding Solar Minima
Authors: Mikic, Z.; Linker, J. A.; Riley, P.; Lionello, R.; Titov,
V. S.
Bibcode: 2009AGUFMSH13C..05M
Altcode:
It is now evident that the present solar minimum is characterized
by unusual properties compared with those of recent solar space-age
minima. In particular, when compared with the previous solar minimum
(which occurred in 1996), the current cycle has an increased incidence
of spotless days, a decreased polar magnetic field strength, decreased
in situ magnetic field measurements at Earth orbit and Ulysses,
and a decreased solar wind pressure. The present minimum is also
characterized by a greater incidence of small equatorial coronal
holes that do not visibly connect with the polar coronal holes. We
compare high-resolution 3D MHD simulations of the corona during these
two intervals to understand the nature of the difference between the
present and previous solar minima. To minimize differences in the
comparison, our simulations are based on magnetic field data measured
with the same instrument, namely the MDI instrument aboard SOHO. We
will use these simulations to explore the detailed characteristics
of the corona during these two periods, with particular emphasis on
understanding the key factors that produce the differences in the
observed properties of the solar corona. Research supported by NASA's
Heliospheric Theory and Living With a Star Programs, and NSF/CISM.
Title: On the Relationship Between Coronal Heating, Magnetic Flux,
and the Density of the Solar Wind
Authors: Riley, P.; Mikic, Z.; Linker, J. A.; McComas, D. J.;
Schwadron, N. A.
Bibcode: 2009AGUFMSH11A1496R
Altcode:
The stark differences between the current solar minimum and the
previous one offer a unique opportunity to develop new constraints on
mechanisms for heating and acceleration of the solar wind. Following
on from studies by McComas et al. (2008) and Schwadron and McComas
(2008), we have used Ulysses measurements made during Ulysses' first
and third orbit southern and northern polar passes (i.e., during
near-solar minimum conditions) to deduce a linear relationship between
proton number density (Np) and radial magnetic field strength (Br) in
the high-speed quiescent solar wind. We have explored ideas that this
could have resulted from interplanetary processes, or the super-radial
expansion of the coronal magnetic field close to the Sun, but find that
neither possibility can produce the observed relationship. Instead,
we suggest that it is a natural consequence of a coronal heating law
of the form Q ~ B. This interpretation is supported by the results of
one-dimensional simulations, which show that Q ~ Np(1 AU) for a range
of heating scale-lengths and field-line geometries. Additionally, our
results suggest a systematic temporal decrease in the polar magnetic
field strength during the 4 solar minimum polar passes: As the polar
magnetic fields have weakened both from the spacecraft's traversal
from southern to northern hemisphere, and from the previous minimum to
the current one, commensurate drops in coronal heating have resulted
in decreases in the density of the polar coronal-hole plasma. We
have also computed the X-ray spectral radiance, which serves as an
independent proxy for the power dissipated through coronal heating,
to assess whether it also scales linearly with B.
Title: MHD Simulations of the Interaction of Small Polarities in
Coronal Holes
Authors: Lionello, R.; Linker, J. A.; Mikic, Z.; Titov, V. S.
Bibcode: 2009AGUFMSH44A..05L
Altcode:
Coronal holes are extended regions of open magnetic field with densities
significantly lower than the typical background corona. Coronal
holes rotate quasi-rigidly in contrast to the underlying photosphere
whose rotation rate has a strong latitudinal dependence. All proposed
explanations of this phenomenon (i.e., that of Wang and Sheeley and that
of Fisk and coworkers) require magnetic reconnection to explain coronal
hole evolution. However, until recently, observational surveys seeking
evidence of such reconnection have been inconclusive. Newer observations
made by the Hinode satellite reveal events (jets) throughout coronal
holes that are strongly suggestive of reconnection. We have used our
MHD model to investigate magnetic reconnection in coronal holes when
for small magnetic field polarities of the same sign as they are pushed
closer by surface flows. We will discuss the topological aspects of
our simulations and the implications for the physics of coronal holes.
Title: Investigating the Topology of the “Disconnection” of
Coronal Holes
Authors: Titov, V. S.; Mikic, Z.; Linker, J. A.; Antiochos, S. K.;
Lionello, R.
Bibcode: 2009AGUFMSH41B1665T
Altcode:
Using a potential-field-source-surface approximation, we construct
an exact analytical model to describe the intrusion of a magnetic
flux spot from the closed-field region into the polar coronal hole
(CH). The spot, which has an opposite polarity compared to the
surrounding field, moves across a local bulge in the CH, eventually
detaching it into a separate minor CH. We show that the formation
of a magnetic minimum point, its subsequent degeneration into a null
point, and its bifurcation into a pair of nulls, plays a key role in
this process. The separatrix field lines that emanate from the nulls
form an interface between the open and closed field structures. This
implies that the corresponding MHD evolution must involve magnetic
reconnection to accommodate the redistribution of their magnetic
fluxes. We anticipate that the reconnection outflows along the open
part of the separatrix field lines may serve as a source of slow solar
wind. Work supported by NASA and the Center for Integrated Space Weather
Modeling (an NSF Science and Technology Center). Topological skeleton
of the magnetic field in the neighborhood of a detached minor coronal
hole; the skeleton consists of separatrix field lines emanating from
two magnetic null points. The gray-shaded photospheric distribution of
the squashing factor depicts the corresponding footprints of separatrix
surfaces and quasi-separatrix layers.
Title: Implications of the Deep Minimum for Slow Solar Wind Origin
Authors: Antiochos, S. K.; Mikic, Z.; Lionello, R.; Titov, V. S.;
Linker, J. A.
Bibcode: 2009AGUFMSH11A1502A
Altcode:
The origin of the slow solar wind has long been one of the most
important problems in solar/heliospheric physics. Two observational
constraints make this problem especially challenging. First, the slow
wind has the composition of the closed-field corona, unlike the fast
wind that originates on open field lines. Second, the slow wind has
substantial angular extent, of order 30 degrees, which is much larger
than the widths observed for streamer stalks or the widths expected
theoretically for a dynamic heliospheric current sheet. We propose
that the slow wind originates from an intricate network of narrow
(possibly singular) open-field corridors that emanate from the polar
coronal hole regions. Using topological arguments, we show that these
corridors must be ubiquitous in the solar corona. The total solar
eclipse in August 2008, near the lowest point of the Deep Minimum,
affords an ideal opportunity to test this theory by using the ultra-high
resolution Predictive Science's (PSI) eclipse model for the corona and
wind. Analysis of the PSI eclipse model demonstrates that the extent
and scales of the open-field corridors can account for both the angular
width of the slow wind and its closed-field composition. We discuss the
implications of our slow wind theory for the structure of the corona
and heliosphere at the Deep Minimum and describe further observational
and theoretical tests. This work has been supported by the NASA HTP,
SR&T, and LWS programs.
Title: Do Closed Field Regions Contribute Plasma to the Slow Solar
Wind?
Authors: Linker, Jon A.; Lionello, Roberto; Mikic, Zoran; Titov,
Viacheslav S.; Antiochos, Spiro
Bibcode: 2009shin.confE.140L
Altcode:
Composition differences between the fast and slow solar wind suggest
that the slow solar wind plasma has a different origin than the fast
wind. A natural way that a bifurcation in the plasma properties could
arise is if the slow wind plasma originates from previously closed field
regions in the corona. I this talk I will discuss arguments both for
and against this idea, and I will illustrate mechanisms by which the
streamer belt can be opened as part of the slow evolution of the corona.
Title: Simulating the May 13, 2005 CME Event
Authors: Mikic, Zoran; Linker, Jon A.; Lionello, Roberto; Riley,
Pete; Titov, Viacheslav
Bibcode: 2009shin.confE..57M
Altcode:
We will describe progress on our 3D MHD simulations of the May 13,
2005 CME Event.
Title: Using MHD Models to Understand CIR Structure
Authors: Riley, Pete; Linker, Jon A.; Mikic, Zoran; Lionello, Roberto
Bibcode: 2009shin.confE..38R
Altcode:
Corotating interaction regions (CIRs) dominate the large-scale,
ambient structure of the inner heliosphere. They result from quasi-
stationary conditions at the Sun, whereby solar rotation leads to
parcels of plasma with different plasma and magnetic properties
becoming radially aligned. This interaction is one of the principal
global dynamic processes that shape the structure of the interplanetary
medium. Global MHD models provide a convenient approach for exploring
the three-dimensional context of in situ observations of CIRs and
understanding how this structure arises. Our model, which includes
energy transport processes, such as coronal heating,conduction of heat
parallel to the magnetic field, radiative losses, and the effects of
Alfven waves, is capable of producing significantly better estimates
of the plasma temperature and density in the corona than have been
possible in the past. With such a model, we can compute emission in
extreme ultraviolet (EUV) and X-ray wavelengths, as well as scattering
in polarized white light. Additionally, from our heliospheric solutions,
we can deduce magnetic field and plasma parameters along specific
spacecraft trajectories. In this talk, we will use MHD model solutions
to review the processes that lead to the formation and evolution of
CIRs, and, in particular, highlight the properties of the current
solar minimum which appear to be unique in a number of respects.
Title: Current Sheet Energetics, Flare Emissions, and Energy Partition
in a Simulated Solar Eruption
Authors: Reeves, Kathy K.; Linker, Jon; Mikic, Zoran; Forbes, Terry
Bibcode: 2009shin.confE.171R
Altcode:
We investigate coronal energy flow during a simulated coronal mass
ejection (CME). We model the CME using a 2.5D numerical MHD code that
includes coronal heating, thermal conduction and radiative cooling
in the energy equation. We apply energy diagnostics to this numerical
simulation and find that the energy is conserved to within 1% during
the stages leading up to the eruption, and to within 5% during the
eruption, when the current sheet is formed. These errors are most likely
due to the additional numerical dissipation associated with numerical
diffusion. We also examine the energy release in the current sheet as
the eruption takes place. We find that Poynting flux is the dominant
carrier of energy into the current sheet region, although its effect is
mitigated by losses due to conductive flux and viscosity. Additionally,
we examine the simulated emissions due to the flare associated with
the CME and find that the simulation reproduces observed flare loop
morphologies and provides an explanation for coronal dimming during
eruptions.
Title: Small Bipoles Interacting with a Coronal Hole: MHD Simulations
Authors: Lionello, Roberto; Linker, Jon A.; Mikic, Zoran; Titov,
Viacheslav S.; Antiochos, Spiro
Bibcode: 2009shin.confE.128L
Altcode:
Coronal holes are known to be the source of the fast wind and are
also believed to play a key role in the formation of the slow wind;
consequently, their evolution is critical for understanding how
the heliospheric magnetic field connects to the Sun. In the context
of field reversal, the Fisk model postulates that open flux can be
transported out of coronal holes into the closed field region through
interchange reconnection with small loops associated with parasitic
polarities. This scenario is supported by in-situ observations,
which seem to favor interchange reconnection as the only mechanism
responsible for field reversal. However, it is hard to reconcile with
theoretical results on the topology of coronal holes. To determine the
feasibility of this mechanism, we have used our 3D MHD algorithm in
spherical coordinates to study the interaction of the magnetic field of
two bipoles with a coronal hole. The model uses a polytropic treatment
for the energy equation and includes a self-consistent solar wind. We
have prescribed as magnetic flux distribution at the lower boundary,
a smoothed Kitt Peak magnetogram for Carrington Rotation 1913 (late
August 1996), to which we have added two small bipoles. After reaching
a relaxed state with well-defined coronal holes and a close field
region inside a helmet streamer, we have introduced surface flows,
which evolve the magnetic flux distribution at the boundary. We have
investigated the reconfiguration of the coronal fields in response to
these motions; in particular we show what happens to the open flux in
the system as the bipoles move from the coronal holes into the closed
field region. We have found no evidence that open flux can be injected
into closed-field regions. Portions of coronal holes that may appear to
have been detached are actually still connected to the main coronal hole
through zero-width corridors. We conclude that interchange reconnection,
by itself, does not produce the open-closed field mixture postulated by
the Fisk model. On the other hand, the magnetic topology of the coronal
hole boundary becomes so complex that some of the essential features
of the model, in particular the open field diffusion, may prove to be
an effective approximation for capturing the magnetic dynamics.
Title: A novel metric for coronal MHD models
Authors: Schmit, D. J.; Gibson, S.; de Toma, G.; Wiltberger, M.;
Hughes, W. J.; Spence, H.; Riley, P.; Linker, J. A.; Mikic, Z.
Bibcode: 2009JGRA..114.6101S
Altcode: 2009JGRA..11406101S
In the interest of quantitatively assessing the capabilities of
coronal MHD models, we have developed a metric that compares the
structures of the white light corona observed with SOHO LASCO C2
to model predictions. The MAS model is compared to C2 observations
from two Carrington rotations during solar cycle 23, CR1913 and
CR1984, which were near the minimum and maximum of solar activity,
respectively, for three radial heights, 2.5 R $\odot$ ,
3.0 R $\odot$ , and 4.5 R $\odot$ . In addition
to simulated polarization brightness images, we create a synthetic
image based on the field topology along the line of sight in the
model. This open-closed brightness is also compared to LASCO C2 after
renormalization. In general, the model's magnetic structure is a
closer match to observed coronal structures than the model's density
structure. This is expected from the simplified energy equations used
in current global corona MHD models.
Title: Coronal Modeling: Present Status and Challenges for the Future*
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.; Titov, V.
Bibcode: 2009AGUSMSH22A..04L
Altcode:
The solar corona strongly influences space weather at Earth, via
eruptive phenomena such as coronal mass ejections, and through its
structure, which leads to the formation of fast solar wind streams
that trigger recurrent geomagnetic activity. MHD models that address
both the dynamics and structure of the solar corona have advanced
considerably in recent years, but many challenges remain if these
models are to provide reliable space weather forecasting tools. In
this talk we describe these challenges and the prospects for meeting
them. *Research supported by NASA, NSF and AFOSR.
Title: Structure of the Erupting Magnetic Field in May 12 1997
CME Event
Authors: Titov, Viacheslav; Mikic, Z.; Linker, J. A.; Lionello, R.
Bibcode: 2009SPD....40.2213T
Altcode:
We have studied the May 12 1997 CME event by combining two
powerful tools: (1) Our numerical MHD model of erupting magnetic
configurations; and (2) Our generalized method for analyzing their
magnetic structure. This approach allows us to identify the building
blocks of such configurations by computing all their separatrix and
quasi-separatrix surfaces that play the role of interfaces between
such blocks. The latter include, in particular, a flare arcade and
erupting magnetic flux rope, which we relate to the observed flare
ribbons and EUV dimmings of the event. This provides a solid basis for
a very detailed comparison of our MHD model with observational data
on this eruption. Such a comparison helps us to verify our model and
understand what physical processes occur at different stages of the
observed event. This work is supported by NASA and the Center
for Integrated Space Weather Modeling (an NSF Science and Technology
Center).
Title: Comparing Modeled EUV and X-Ray Emission from the Global
Corona with Observations
Authors: Mikic, Zoran; Linker, J. A.; Lionello, R.; Riley, P.;
Titov, V.
Bibcode: 2009SPD....40.1402M
Altcode:
Comparing emission in EUV and X-ray wavelengths from global coronal
models provides a very sensitive constraint on coronal heating
models. The ability of synthetic emission estimates to discriminate
between different models increases considerably when observations
are taken in many spectral lines. We will compare emission from our
global 3D MHD model of the corona with EUV emission from EIT/SOHO
and EUVI/SECCHI/STEREO, as well as soft X-rays from Yohkoh SXT and
Hinode XRT. These constrains will help to improve our coronal heating
model. Research supported by NASA's Living With a Star Program,
NASA's Heliospheric Theory Program, and NSF/CISM.
Title: Computing Emissions from Active-Region Loops in 3D and High
Resolution
Authors: Mok, Yung; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2009SPD....40.1208M
Altcode:
Plasma loops are widely observed in EUV and soft X-ray over active
regions, but their thermal properties and formation mechanism have
be controversial. In this work, we are able to reproduce some of
the loop properties by forward modeling. Using an MDI magnetogram,
we constructed a mildly sheared force-free magnetic field based on
parameters deduced from observation. The field was computed in unusually
high spatial resolution in order to resolve the expected thin coronal
loops. Although the magnetogram has fine structures at the photospheric
level, the field in the corona is smooth as expected. The field lines
have moderately complex connectivity. We then chose a specific heating
model and computed the thermal structure in 3D. Although the overall
temperature profile has only moderate spatial variations in the corona,
the computed line-of-sight integrated EUV emissions show a complex
system of thin plasma loops. Initial analysis shows that thermal
instability leads to the time variation of the loop brightness. The
lack of cross-section expansion is also apparent. The location of
the loops and their relationship with the magnetic field will also be
discussed. Work supported by HTP of NASA. Computation resources
provided by NAS at Ames Research Center, NASA.
Title: Relating Emission to Magnetic Field Evolution in Eruptive
Phenomena
Authors: Linker, Jon A.; Mikic, Z.; Lionello, R.; Titov, V.; Riley, P.
Bibcode: 2009SPD....40.2203L
Altcode:
The magnetohydrodynamic (MHD) equations are frequently used to
investigate coronal mass ejections (CMEs), eruptive prominences, and
solar flares. The proposed mechanisms responsible for these phenomena
are under still under vigorous debate. Because most MHD models use
relatively simple energy equations, the debate between theorists often
centers on the interpretation and comparison of magnetic field evolution
in the models with corresponding features observed in emission. On the
other hand, observers often look at emission images and try to deduce
what magnetic evolution is occurring. We have developed MHD models
that include energy transport (radiative losses, anisotropic thermal
conduction, and coronal heating) in the transition region and solar
corona. This more accurate representation of energy flow allows us to
compute simulated EUV and X-ray emission and compare directly with
observations. In this paper we will show examples of this modeling
approach for specific CME events and describe the magnetic field
evolution associated with commonly observed emission features such
as dimming regions and postflare loops. Work supported by NASA,
AFOSR, and the Center for Integrated Space Weather Modeling (an NSF
Science and Technology Center).
Title: Slip-Squashing Factors as a Measure of Three-Dimensional
Magnetic Reconnection
Authors: Titov, V. S.; Forbes, T. G.; Priest, E. R.; Mikić, Z.;
Linker, J. A.
Bibcode: 2009ApJ...693.1029T
Altcode: 2008arXiv0807.2892T
A general method for describing magnetic reconnection in arbitrary
three-dimensional magnetic configurations is proposed. The method
is based on the field-line mapping technique previously used only
for the analysis of a magnetic structure at a given time. This
technique is extended here so as to analyze the evolution of a
magnetic structure. Such a generalization is made with the help of
new dimensionless quantities called "slip-squashing factors." Their
large values define the surfaces that border the reconnected or
to-be-reconnected magnetic flux tubes for a given period of time
during the magnetic evolution. The proposed method is universal,
since it assumes only that the time sequence of evolving magnetic
field and the tangential boundary flows are known. The application
of the method is illustrated for simple examples, one of which was
considered previously by Hesse and coworkers in the framework of the
general magnetic reconnection theory. The examples help us to compare
these two approaches; it reveals also that, just as for magnetic null
points, hyperbolic and cusp minimum points of a magnetic field serve
as favorable sites for magnetic reconnection. The new method admits a
straightforward numerical implementation and provides a powerful tool
for the diagnostics of magnetic reconnection in numerical models of
solar-flare-like phenomena in space and laboratory plasmas.
Title: Multispectral Emission of the Sun During the First Whole Sun
Month: Magnetohydrodynamic Simulations
Authors: Lionello, Roberto; Linker, Jon A.; Mikić, Zoran
Bibcode: 2009ApJ...690..902L
Altcode:
We demonstrate that a three-dimensional magnetohydrodynamic (MHD)
simulation of the corona can model its global plasma density and
temperature structure with sufficient accuracy to reproduce many of the
multispectral properties of the corona observed in extreme ultraviolet
(EUV) and X-ray emission. The key ingredient to this new type of global
MHD model is the inclusion of energy transport processes (coronal
heating, anisotropic thermal conduction, and radiative losses) in the
energy equation. The calculation of these processes has previously been
confined to one-dimensional loop models, idealized two-dimensional
computations, and three-dimensional active region models. We refer
to this as the thermodynamic MHD model, and we apply it to the time
period of Carrington rotation 1913 (1996 August 22 to September
18). The form of the coronal heating term strongly affects the plasma
density and temperature of the solutions. We perform our calculation
for three different empirical heating models: (1) a heating function
exponentially decreasing in radius; (2) the model of Schrijver et al.;
and (3) a model reproducing the heating properties of the quiet Sun and
active regions. We produce synthetic emission images from the density
and temperature calculated with these three heating functions and
quantitatively compare them with observations from EUV Imaging Telescope
on the Solar and Heliospheric Observatory and the soft X-ray telescope
on Yohkoh. Although none of the heating models provide a perfect match,
heating models 2 and 3 provide a reasonable match to the observations.
Title: Global MHD Modeling of the Solar Corona and Inner Heliosphere
for the Whole Heliosphere Interval
Authors: Riley, P.; Lionello, R.; Linker, J. A.; Mikic, Z.
Bibcode: 2008AGUFMSH21C..02R
Altcode:
Whole Heliosphere Interval (WHI), which runs from March 20 through
April 16, 2008, and coincides with Carrington Rotation (CR) 2068 is
providing a unique opportunity for both observers and modelers to
collaborate in an effort to understand the three-dimensional structure
and evolution of the solar corona and inner heliosphere. It builds
on several previous "Whole Sun Month" intervals, which proved to
be exceptionally successful. In support of WHI, we have developed
a global MHD model solution for CR 2068. Our model, which includes
energy transport processes, such as coronal heating, conduction of heat
parallel to the magnetic field, radiative losses, and the effects of
Alfven waves, is capable of producing significantly better estimates
of the plasma temperature and density in the corona than have been
possible in the past. With such a model, we can compute emission in
extreme ultraviolet (EUV) and X-ray wavelengths, as well as scattering
in polarized white light. Additionally, from our heliospheric solutions,
we can deduce magnetic field and plasma parameters along specific
spacecraft trajectories. In this presentation, we make detailed
comparisons of both remote solar and in situ observations with the
model results. Such comparisons allow us and (3) Provide support for
(or against) assumptions in the MHD model, such as which physical
processes are (or are not) important. The results of these simulations
(including post-processing analysis and visualization tools) will be
made available to the scientific community at http://predsci.com/WHI.
Title: Comparing Eclipse Observations of the August 1, 2008 Solar
Corona with an MHD Model Prediction
Authors: Rusin, V.; Mikic, Z.; Aniol, P.; Druckmuller, M.; Saniga,
M.; Linker, J. A.; Lionello, R.; Riley, P.; Titov, V.
Bibcode: 2008AGUFMSH13B1524R
Altcode:
Total solar eclipses offer a unique opportunity to study the white
light and emission coronae at high resolution. Newly developed
image-processing techniques allow us to combine many individual coronal
images with different exposures to produce coronal images during
an eclipse that resemble those taken with radially graded filters,
but with a higher quality. In a separate effort, magnetohydrodynamic
(MHD) models have been used to predict the structure of the corona
prior to eclipses, using measurements of photospheric magnetic fields
on the Sun. In particular, such an MHD model was used to predict the
structure of the corona for the August 1, 2008 total solar eclipse. The
eclipse was observed from Altaj village, Mongolia, under perfect seeing
conditions. The white-light corona was observed with 6 telescopes, with
a focal lenses ranging from 200 mm to 1250 mm. The emission corona at
530.3 nm (Fe XIV) was imaged thorough a narrow passband filter with a
transmission width of 0.03 nm. To separate out the 530.3 nm corona, the
scattered background was substracted from a white-light coronal image
taken at 529.1 nm, taken simultaneously with another narrow passband
filter with a transmission width of 0.03 nm. This was the first time
that the green emission corona was observed during an eclipse. We
will compare the observed images with features from the predicted MHD
model, including magnetic field line traces and simulated polarization
brightness images. Research partially supported by NASA and NSF.
Title: Small Bipoles Interacting with a Coronal Hole: MHD Simulations
Authors: Lionello, R.; Linker, J. A.; Mikić, Z.
Bibcode: 2008AGUFMSH51B1602L
Altcode:
Changes in the photospheric magnetic flux due to emergence, submergence,
and surface flows drive the evolution of the coronal and heliospheric
magnetic field. We have used our 3D MHD algorithm in spherical
coordinates to study the interaction of the magnetic field of two
bipoles with a coronal hole. We have prescribed as magnetic flux
distribution at the lower boundary a smoothed Kitt Peak magnetogram
for Carrington Rotation 1913 (late August 1996), to which we have added
two small bipoles. After reaching a relaxed state, we have introduced
surface flows, which evolve the magnetic flux distribution at the
boundary. We have investigated the reconfiguration of coronal fields
in response to these motions; in particular we show what happens to the
open flux associated with the bipole when it is moved into a previously
closed region.
Title: Slip-Squashing Factors as a Measure of Three-Dimensional
Magnetic Reconnection
Authors: Titov, V. S.; Forbes, T. G.; Priest, E. R.; Mikic, Z.;
Linker, J. A.
Bibcode: 2008AGUFMSM31A1713T
Altcode:
A general method for describing magnetic reconnection in arbitrary
three-dimensional magnetic configurations is proposed. The method is
based on the field-line mapping technique previously used only for
the analysis of magnetic structure at a given time. This technique is
extended here so as to analyze the evolution of magnetic structure. Such
a generalization is made with the help of new dimensionless quantities
called "slip-squashing factors". Their large values define the surfaces
that border the reconnected or to-be-reconnected magnetic flux tubes
for a given period of time during the magnetic evolution. The proposed
method is universal, since it assumes only that the time sequence
of the evolving magnetic field and the tangential boundary flows are
known. The application of the method is illustrated for simple examples,
one of which was considered previously by Hesse and coworkers in the
framework of the general magnetic reconnection theory. The examples
help to compare these two approaches; they reveal also that, just as for
magnetic null points, hyperbolic and cusp minimum points of a magnetic
field may serve as favorable sites for magnetic reconnection. The new
method admits a straightforward numerical implementation and provides
a powerful tool for the diagnostics of magnetic reconnection in
numerical models of solar-flare-like phenomena in space and laboratory
plasmas. Research partially supported by NASA and NSF.
Title: Is Disconnection Necessary?
Authors: Linker, J. A.; Lionello, R.; Mikic, Z.; Riley, P.; Titov,
V. S.
Bibcode: 2008AGUFMSH43B..08L
Altcode:
A number of diverse observations indicate that at least a portion
of the slow solar wind plasma must originate from regions that were
at one time magnetically closed (e.g., within the helmet streamer
belt). The release of this material could result from closed fields
expanding outward, balanced by disconnection of previously open fields,
or it could occur through interchange reconnection, where open field
lines reconnect with previously closed field lines. The scarcity of
evidence for disconnection in interplanetary measurements has led to
the idea that reconfiguration of coronal fields must occur entirely
through interchange reconnection (e.g. Fisk and Schwadron 2001 ApJ
560, 425). We have performed several time-dependent MHD simulations of
coronal evolution in the presence of surface flows such as differential
rotation and the motion of small bipoles. We find that all three
processes (disconnection, interchange reconnection, and opening of
previously closed loops) occur. We discuss the implications of our
work for the origin of the slow solar wind. Research supported by NASA,
NSF (through CISM and the Strategic Capabilities Program) and AFOSR.
Title: Global MHD Modeling of the Solar Wind and CMEs: Energetic
Particle Applications
Authors: Riley, Pete; Linker, Jon A.; Mikic, Zoran; Lionello, Roberto
Bibcode: 2008AIPC.1039..279R
Altcode:
Global MHD models of Coronal Mass Ejections (CMEs) can provide
important insights into the physical processes associated with the
eruption and evolution of CMEs and the acceleration of SEPs, and are a
valuable tool for interpreting both remote solar and interplanetary in
situ observations. Moreover, they represent a virtual laboratory for
exploring conditions and regions of space that are not conveniently or
currently accessible by spacecraft. The most energetic events typically
originate from active regions on the Sun. To accurately model such
regions, whilst also capturing the global corona, requires an MHD
model that includes energy transport (radiative losses, anisotropic
thermal conduction, and coronal heating) in the transition region
and corona. Equally importantly, the model must reproduce an accurate
ambient solar wind through which the CME propagates. In this report,
we describe the current status of modeling efforts, and present three
applications that we believe are relevant in studies of energetic
particles: the Alfvén speed in the corona; the evolution of the
heliospheric current sheet; and CME eruptions.
Title: The Formation of Coronal Loops by Thermal Instability in
Three Dimensions
Authors: Mok, Yung; Mikić, Zoran; Lionello, Roberto; Linker, Jon A.
Bibcode: 2008ApJ...679L.161M
Altcode:
Plasma loops in solar active regions have been observed in EUV and
soft X-rays for decades. Their formation mechanism and properties,
however, are still not fully understood. Predictions by early models,
based on 1D hydrostatic equilibria with uniform plasma heating, are
not consistent with high-resolution measurements. In this Letter,
we demonstrate, via 3D simulations, that a class of heating models
can lead to the dynamic formation of plasma loops provided the plasma
is heated sufficiently to match SXT soft X-ray measurements. We show
that individual flux tubes in a 3D magnetic structure tend to stand out
against their neighbors. The loops have large aspect ratios and nearly
uniform cross sections in the corona, similar to those observed by
EIT and TRACE. The coronal EUV emission from these thermally unstable
solutions is roughly consistent with EIT measurements. The solution
oscillates in time through a large-amplitude, nonlinear cycle, leading
to repeated brightening and fading of the loops.
Title: Exploring the CME-ICME Connection via CME Event Studies
Authors: Mikic, Z.; Linker, J. A.; Lionello, R.; Riley, P.; Titov,
V.; Odstrcil, D.
Bibcode: 2008AGUSMSP24A..06M
Altcode:
We describe how numerical simulations of CME initiation and propagation
in the heliosphere can be used to study the connection between coronal
mass ejections and interplanetary magnetic clouds. The detailed
relationship between the plasma and magnetic fields in solar active
regions, the source regions of CMEs, and subsequent in situ measurements
in interplanetary magnetic clouds, can best be studied using "CME
event studies." These attempt to model a particular CME in the greatest
possible detail, including the use of measured photospheric magnetic
fields, to explain observations. We will describe our event studies
of the May 12, 1997 SHINE CME Event, as well as the May 13, 2005 LWS
Focused Science Topic CME Event. Research supported by NASA's Living
With a Star Program, NASA's Heliospheric Theory Program, and NSF/CISM.
Title: A Determination of the Value and Variability of the Sun's
Open Magnetic Flux using a Global MHD Model
Authors: Riley, P.; Mikic, Z.; Linker, J.; Harvey, J. W.; Hoeksema,
T.; Liu, Y.; Bertello, L.
Bibcode: 2008AGUSMSH44A..03R
Altcode:
The underlying value and variation of the Sun's open, unsigned magnetic
flux is of fundamental scientific importance, yet its properties remain
poorly known. For example, do long term (on the time-scale of ~ 100
years) changes in the strength of the solar magnetic field exist and
do they persist through the heliosphere? If present, they may have
a direct impact on space climate, including implications for the
transport of cosmic rays (CRs), and as such, may affect technology,
space, and even terrestrial climate. Global MHD models are capable of
reproducing the structure of the large-scale solar and interplanetary
magnetic field (at least in the absence of transient phenomena such as
Coronal Mass Ejections), and should, in principle, be able to address
this topic. However, they rely - and depend crucially - on boundary
conditions derived from observations of the photospheric magnetic
field. In spite of ~ 40 years of measurements, accurate estimates of the
radial component of the photospheric magnetic field remain difficult
to make. In this study, we attempt to find a "ground truth" estimate
of the photospheric magnetic field by carefully comparing both disk
magnetograms and diachronic (previously known as synoptic) maps from
6 different observatories (KPVT, SOLIS, GONG, MDI, WSO, and MWO). We
find that although there is a general consensus between several of
them, there are also some significant discrepancies. Using data from
these observatories, we compute global heliospheric solutions for a
selection of epochs during the last 3 solar cycles and compare the
results with in situ observations. We apply these results to several
topics related to the Sun's open flux.
Title: Global MHD Modeling of the Solar Corona and Inner Heliosphere
for the Whole Heliosphere Interval
Authors: Riley, P.; Lionello, R.; Linker, J. A.; Mikic, Z.
Bibcode: 2008AGUSMSH53A..06R
Altcode:
Whole Heliosphere Interval (WHI), which runs from March 20 through
April 16, 2008, and coincides with Carrington Rotation (CR) 2068
will provide a unique opportunity for both observers and modelers to
collaborate in an effort to understand the three-dimensional structure
and evolution of the solar corona and inner heliosphere. It builds
on several previous "Whole Sun Month" intervals, which proved to
be exceptionally successful. In support of WHI, we will develop
a preliminary global MHD model solution for CR 2068. Our model,
which includes energy transport processes, such as coronal heating,
conduction of heat parallel to the magnetic field, radiative losses,
and the effects of Alfven waves, is capable of producing significantly
better estimates of the plasma temperature and density in the corona
than have been possible in the past. With such a model, we can compute
emission in extreme ultraviolet (EUV) and X-ray wavelengths, as well as
scattering in polarized white light. Additionally, from our heliospheric
solutions, we can deduce magnetic field and plasma parameters along
specific spacecraft trajectories. Detailed comparisons of both remote
solar and in situ observations with the model results will allow us
to: (1) Connect these disparate set of observations; (2) Infer the
global structure of the inner heliosphere; (3) Provide support for (or
against) assumptions in the MHD model; and (4) Explore the effects of
poorly understood physical processes, such as energy transport and/or
differential rotation. We will make the results of these simulations
(including post-processing analysis and visualization tools) available
to the scientific community at iMHD.net/WHI.
Title: Understanding the Nature of "EIT" Waves
Authors: Linker, J. A.; Lionello, R.; Mikic, Z.; Titov, V.; Riley, P.
Bibcode: 2008AGUSMSP31D..05L
Altcode:
Since the first observations of EIT waves in the May 1997 CME event
(Thompson et al. 1998), their origin and nature has been the subject
of considerable debate. Explanation for the signatures include a fast
mode shock (Uchida, 1968), fast mode waves (Wang 2000, Wu et al. 2001,
Ofman & Thompson 2002, Warmuth et al. 2004, Ofman 2007), solitons
(Wills-Davey et al. 2007), and opening of the magnetic field associated
with the erupting CME (Attril et al. 2007, Delannee et al. 2008). We
have developed MHD models that include energy transport (radiative
losses, anisotropic thermal conduction, and coronal heating) in the
transition region and solar corona. This more accurate representation
of energy flow allows us to compute simulated EUV and X-ray emission
and compare directly with observations. We have developed an MHD
simulation of the May 97 event, with many characteristics of the
actual event, including waves that appear in simulated EIT images. We
discuss the analysis of these waves in relation to popular hypotheses
for EIT waves. Work supported by NASA and CISM, an NSF Science and
Technology Center.
Title: Comparison with Observations of EUV and X-Ray Emissions
Calculated with Different Heating Models for Active Region 7986
Authors: Lionello, R.; Linker, J. A.; Mikić, Z.; Mok, Y.
Bibcode: 2008AGUSMSP31A..03L
Altcode:
We have calculated the EUV and X-Ray emissions for AR 7986 in late
August 1996 for different heating models using three-dimensional
simulations of the magnetic field and of the thermal structure. From our
simulations we have extracted time-sequences of emissions from different
point of views and compared them quantitatively with the observations
from SoHo EIT and Yohkoh SXT. Work supported by NASA LWS program.
Title: The Sun as the Source of Heliospheric "Space Weather": A CISM
Integrated Model Perspective and STEREO Inspiration
Authors: Luhmann, J. G.; Li, Y.; Lynch, B.; Lee, C. O.; Huttunen, E.;
Liu, Y.; Toy, V.; Odstrcil, D.; Riley, P.; Linker, J.; Mikic, Z.; Arge,
C.; Petrie, G.; Zhao, X.; Liu, Y.; Hoeksema, T.; Owens, M.; Galvin,
A.; Simunac, K.; Howard, R.; Vourlidas, A.; Jian, L. K.; Russell, C. T.
Bibcode: 2008AGUSMSH31C..01L
Altcode:
Models developed under the Center for Integrated Space weather
Modeling (CISM) represent one effort that is underway to realistically
simulate the Sun's physical controls over interplanetary conditions,
or heliospheric "space weather", in three dimensions. This capability
is critical for interpreting the latest observations from STEREO,
whose goal is to enable connections to be made between what is
observed in the heliosphere via distributed in-situ measurements
and what is observed in the corona and heliosphere via imaging from
separated 1 AU perspectives. The ways in which the CISM models are
enabling the exploitation of STEREO and other observations toward
increased understanding of the solar wind and coronal activity and
its consequences are described. In particular, the models allow the
identification of the sources of structures in the solar wind, and
analyses of how the coronal context of the observed CMEs plays a key
role in determining the ultimate terrestrial (and other planetary)
response .
Title: Numerical Simulations of Solar Wind Disturbances by Coupled
Models
Authors: Odstrcil, D.; Pizzo, V. J.; Arge, C. N.; Bissi, M. M.; Hick,
P. P.; Jackson, B. V.; Ledvina, S. A.; Luhmann, J. G.; Linker, J. A.;
Mikic, Z.; Riley, P.
Bibcode: 2008ASPC..385..167O
Altcode:
Numerical modeling plays a critical role in efforts to understand the
connection between solar eruptive phenomena and their impacts in the
near-Earth space environment and in interplanetary space. Coupling the
heliospheric model with empirical, observational, and numerical coronal
models is described. Results show background solar wind, evolution
of interplanetary transients, connectivity of magnetic field lines,
and interplanetary shocks approaching geospace.
Title: Theoretical modeling for the stereo mission
Authors: Aschwanden, Markus J.; Burlaga, L. F.; Kaiser, M. L.; Ng,
C. K.; Reames, D. V.; Reiner, M. J.; Gombosi, T. I.; Lugaz, N.;
Manchester, W.; Roussev, I. I.; Zurbuchen, T. H.; Farrugia, C. J.;
Galvin, A. B.; Lee, M. A.; Linker, J. A.; Mikić, Z.; Riley, P.;
Alexander, D.; Sandman, A. W.; Cook, J. W.; Howard, R. A.; Odstrčil,
D.; Pizzo, V. J.; Kóta, J.; Liewer, P. C.; Luhmann, J. G.; Inhester,
B.; Schwenn, R. W.; Solanki, S. K.; Vasyliunas, V. M.; Wiegelmann, T.;
Blush, L.; Bochsler, P.; Cairns, I. H.; Robinson, P. A.; Bothmer,
V.; Kecskemety, K.; Llebaria, A.; Maksimovic, M.; Scholer, M.;
Wimmer-Schweingruber, R. F.
Bibcode: 2008SSRv..136..565A
Altcode: 2006SSRv..tmp...75A
We summarize the theory and modeling efforts for the STEREO mission,
which will be used to interpret the data of both the remote-sensing
(SECCHI, SWAVES) and in-situ instruments (IMPACT, PLASTIC). The
modeling includes the coronal plasma, in both open and closed magnetic
structures, and the solar wind and its expansion outwards from the Sun,
which defines the heliosphere. Particular emphasis is given to modeling
of dynamic phenomena associated with the initiation and propagation
of coronal mass ejections (CMEs). The modeling of the CME initiation
includes magnetic shearing, kink instability, filament eruption, and
magnetic reconnection in the flaring lower corona. The modeling of CME
propagation entails interplanetary shocks, interplanetary particle
beams, solar energetic particles (SEPs), geoeffective connections,
and space weather. This review describes mostly existing models of
groups that have committed their work to the STEREO mission, but is by
no means exhaustive or comprehensive regarding alternative theoretical
approaches.
Title: Strongly compressible current sheets under gravitation
Authors: Vainshtein, S. I.; Mikic, Z.; Rosner, R.; Sagdeev, R. Z.
Bibcode: 2008arXiv0804.3789V
Altcode:
Many stormy events in astrophysics occur due to the sudden magnetic
energy release. This is possible if a magnetic configuration abruptly
changes its topology, an event usually referred to as magnetic
reconnection. It is known that pure Ohmic decay is inefficient,
occurring during cosmological times (due to the huge characteristic
scales $L$). It is recognized that the presence of current sheets speeds
up the process, but still insufficiently$^{1,2,3,4,5}$. We show that,
in highly compressible and substantially gravitational media, the
reconnection is fast enough to account for stormy events. Thus, highly
compressible situations offer exiting opportunities in explanations
of violent events, although full-scale compressible and gravitational
simulations proved to be quite challenging.
Title: 1997 May 12 Coronal Mass Ejection Event. I. A Simplified
Model of the Preeruptive Magnetic Structure
Authors: Titov, V. S.; Mikic, Z.; Linker, J. A.; Lionello, R.
Bibcode: 2008ApJ...675.1614T
Altcode: 2007arXiv0711.3801T
A simple model of the coronal magnetic field prior to the coronal mass
ejection (CME) eruption on 1997 May 12 is developed. First, the magnetic
field is constructed by superimposing a large-scale background field
and a localized bipolar field to model the active region (AR) in the
current-free approximation. Second, this potential configuration is
quasi-statically sheared by photospheric vortex motions applied to
two flux concentrations of the AR. Third, the resulting force-free
field is then evolved by canceling the photospheric magnetic flux
with the help of an appropriate tangential electric field applied to
the central part of the AR. To understand the structure of the modeled
configuration, we use the field line mapping technique by generalizing
it to spherical geometry. We demonstrate that the initial potential
configuration contains a hyperbolic flux tube (HFT) which is a union
of two intersecting quasi-separatrix layers. This HFT provides a
partition of the closed magnetic flux between the AR and the global
solar magnetic field. Such a partition is approximate since the entire
flux distribution is perfectly continuous. The vortex motions applied
to the AR interlock the field lines in the coronal volume to form
additionally two new HFTs pinched into thin current layers. Reconnection
in these current layers helps to redistribute the magnetic flux and
current within the AR in the flux-cancellation phase. In this phase,
a magnetic flux rope is formed together with a bald patch separatrix
surface wrapping around the rope. Other important implications of
the identified structural features of the modeled configuration are
also discussed.
Title: Using Global Simulations to Relate the Three-Part Structure
of Coronal Mass Ejections to In Situ Signatures
Authors: Riley, Pete; Lionello, Roberto; Mikić, Zoran; Linker, Jon
Bibcode: 2008ApJ...672.1221R
Altcode:
White-light observations of coronal mass ejections (CMEs) often show
the classic "three-part" structure consisting of (1) a bright front; (2)
a dark cavity; and (3) a bright, compact core. It has proven difficult
to unambiguously associate these features with in situ measurements of
interplanetary CMEs (ICMEs), in all but a few cases. In this study we
use a global MHD model to simulate the eruption and evolution of a CME
out to 0.25 AU, allowing us to continuously track these features from
the Sun and through the solar wind. Our results support the generally
held view that the interplanetary flux rope corresponds to the dark
cavity. We find that the bright front merges with solar wind material
swept up by the ICME. Thus, the sheath material found ahead of fast
ejecta is in fact composed from both ambient solar wind material,
as well the bright front. We also note that, in this simulation, the
bright front is formed from the overlying streamer configuration from
within which the CME erupted and is not itself coronal material swept
up during the early phase of the eruption. The conclusions reached
in this study are undoubtedly sensitive to the initial configuration
and mechanism used to initiate the CME, and thus care should be taken
when using them to interpret specific observations. On the other hand,
they provide a unique, unbroken connection between remote solar and
interplanetary observations. Ultimately, detailed comparisons between
observations and simulation results may be able to constrain or even
rule out some mechanisms of CME initiation.
Title: What Models Tell Us About The Relationship Between CME Dynamics
and Current Sheet Energetics
Authors: Reeves, Kathy; Linker, Jon; Forbes, Terry; Mikic, Zoran
Bibcode: 2008cosp...37.2584R
Altcode: 2008cosp.meet.2584R
Observers have noted correlations between peak CME acceleration and the
rise of soft X-ray flare emission. One reason given for this observation
is that acceleration of the flux rope and the thermal energy release
rate are are both consequences of the reconnection process, and should
thus have similar time profiles. CME models provide useful frameworks
for examining this question in detail. Using an analytical CME model, we
find that the correlation between thermal energy release rate and flux
rope acceleration is good for cases where the background magnetic field
is high and the reconnection rate is fast, but that the correlation is
poor for cases with low background magnetic fields and slow reconnection
rates. In this analytical model, we assume that the Poynting flux into
the current sheet is completely thermalized. We re-examine these results
in light of recent 2.5D numerical simulations aimed at understanding
the energy balance in the current sheet during an eruption.
Title: Understanding Eruptive Phenomena with Thermodynamic MHD
Simulations
Authors: Linker, Jon; Mikic, Zoran; Riley, Pete; Lionello, Roberto;
Titov, Viacheslav
Bibcode: 2008cosp...37.1786L
Altcode: 2008cosp.meet.1786L
Understanding Eruptive Phenomena with Thermodynamic MHD Simulations
The magnetohydrodynamic (MHD) equations are frequently used to
investigate coronal mass ejections, eruptive prominences, and solar
flares. A key goal of such studies is to deduce how energy stored
in the magnetic field is suddenly released to drive these phenomena,
for which the proposed mechanism(s) is (are) under still under vigorous
debate. Because most MHD models use relatively simple energy equations,
the discussion often centers on the interpretation and comparison of
magnetic field evolution in the models with corresponding features
observed in emission. With new capabilities to study X-ray and EUV
emission from Hinode, as well as complementary observations from
STEREO and SOHO, it now becomes imperative that models advance to
more quantitative comparisons with emission measurements. We have
developed MHD models that include energy transport (radiative losses,
anisotropic thermal conduction, and coronal heating) in the transition
region and solar corona. We refer to this approach as "Thermodynamic
MHD." This more accurate representation of energy flow allows us to
compute simulated EUV and X-ray emission and compare directly with
observations. In this talk we will show examples of this modeling
approach for specific events and describe the magnetic field evolution
associated with commonly observed emission features such as dimming
regions and postflare loops. Work supported by NASA and the Center
for Integrated Space Weather Modeling (an NSF Science and Technology
Center).
Title: Characterization of the slow solar wind in the outer corona
Authors: Abbo, Lucia; Dodero, Maria Adele; Mikic, Zoran; Riley, Pete;
Antonucci, Ester
Bibcode: 2008cosp...37....8A
Altcode: 2008cosp.meet....8A
The study concerns the streamer belt observed during the minimum of
solar activity with the Ultraviolet Coronagraph Spectrometer (UVCS)
onboard SOHO. On the basis of a spectroscopic analysis of the O
VI doublet and HI Ly α lines, the solar wind plasma parameters are
inferred in the extended corona. The analysis accounts for the coronal
magnetic topology, extrapolated through a 3D magneto-hydrodynamic
model, in order to define the streamer boundary and to analyse the
edges of coronal holes. The results of the analysis allow an accurate
identification of the source regions of the slow coronal wind that
are confirmed to be along the streamer boundary in the open magnetic
field region.
Title: Understanding Eruptive Phenomena in the Hinode Era
Authors: Linker, J. A.; Lionello, R.; Mikic, Z.; Riley, P.; Titov, V.
Bibcode: 2007AGUFMSH51C..01L
Altcode:
The magnetohydrodynamic (MHD) equations are frequently used to
investigate coronal mass ejections, eruptive prominences, and solar
flares. A key goal of such studies is to deduce how energy stored in
the magnetic field is suddenly released to drive these phenomena,
for which the proposed mechanism(s) is (are) still under vigorous
debate. Because most MHD models use relatively simple energy equations,
the discussion often centers on the interpretation and comparison of
magnetic field evolution in the models with corresponding features
observed in emission. With new capabilities to study X-ray and EUV
emission from Hinode, as well as complementary observations from
STEREO and SOHO, it now becomes imperative that models advance to
more quantitative comparisons with emission measurements. We have
developed MHD models that include energy transport (radiative losses,
anisotropic thermal conduction, and coronal heating) in the transition
region and solar corona. This more accurate representation of energy
flow allows us to compute simulated EUV and X-ray emission and compare
directly with observations. In this talk we will show examples of this
modeling approach for specific events and describe the magnetic field
evolution associated with commonly observed emission features such as
dimming regions and postflare loops. Work supported by NASA, NSF and
the Center for Integrated Space Weather Modeling (an NSF Science and
Technology Center).
Title: Slow Solar Wind Formation Beyond the Cusp of an Helmet Streamer
Authors: Rappazzo, A. F.; Velli, M.; Liewer, P.; Lionello, R.; Mikic,
Z.; Einaudi, G.; Dahlburg, R.
Bibcode: 2007AGUFMSH21A0290R
Altcode:
The region beyond the cusp of an helmet streamer is characterized by
the presence of a current sheet embedded in a plasma flow. In previous
3D and 2D simulations the velocity has been found, at a fixed radius,
to grow from a slow value at the current sheet towards higher values
towards the polar regions. The steady-state which was reached showed
the bimodal characteristic of the solar wind, but the slow component
did not show its characteristic variability. The velocity profile of
the slow component is in fact not steady in time, and plasma density
enhancements have been observed by the Large-Angle Spectrometric
Coronagraph (LASCO) instrument on board the Solar and Heliospheric
Observatory (SOHO). We present numerical simulations, performed with
the SAIC MHD spherical code (MAS), of the region beyond the cusp of
an helmet streamer from 1 R\odot up to 20 R\odot. With a sufficiently
high resolution magnetic reconnection of the heliospheric current sheet
is observed, which leads to the formation of density-enhanced magnetic
islands that are accelerated radially outward. The reconnection process
also gives rise to an acceleration profile that is not steady in time.
Title: Using Global MHD Models to Interpret STEREO Observations
Authors: Riley, P.; Mikic, Z.; Linker, J. A.; Odstrcil, D.; Luhmann,
J. G.; Vourlidas, A.
Bibcode: 2007AGUFMSH32A0788R
Altcode:
The STEREO mission presents a unique opportunity to combine both
remote and in situ observations from multiple vantage points,
and, in particular, to provide simultaneous limb and disk-centered
observations. In spite of this wealth of data, the system remains
remarkably under-sampled, and reconstructing 3-D structure from
the observations remains a formidable task. In this presentation we
use global MHD models, which reproduce the eruption and evolution of
specific CME events through the corona and past the orbit of Earth, to
explore the relationship between the various remote sensing and in situ
observations that would be seen at the two STEREO spacecraft. These
simulations were developed as part of our contribution to NASA's
Living With a Star TR&T focused science topic on the relationship
between ICMEs and their solar sources. Our model, which incorporates
coronal heating, thermal conduction, and radiation, is capable of
reproducing a wide variety of measurements, ranging from (polarized)
brightness and emission images to in situ time series of magnetic
and plasma parameters. Moreover, we can construct quantities that
are either not directly observable, or can only be inferred locally,
such as the Alfven speed, the location of the heliospheric current
sheet, and the three-dimensional topology of the magnetic field. We
believe that the combination of sophisticated modeling results and
the exciting new measurements from the STEREO mission will allow us
to address fundamental questions concerning the origin and evolution
of CMEs, ultimately allowing us to develop predictive capabilities
related to their potential to generate space weather effects.
Title: Coronal Mass Ejection Initiation and Complex Topology
Configurations in the Flux Cancellation and Breakout Models
Authors: Amari, T.; Aly, J. J.; Mikic, Z.; Linker, J.
Bibcode: 2007ApJ...671L.189A
Altcode:
We present some new results showing that the flux cancellation
model for coronal mass ejections (CMEs) works well also in a
complex-topology magnetic field. We consider as a model problem
the case of the flux-cancellation-driven evolution of a quadrupolar
configuration. We find that (1) during the first phase, the field
evolves slowly, with a twisted flux rope in equilibrium being
created at some time; (2) nonequilibrium sets in at a critical time
and the configuration experiences a major global disruption. These
features are similar to those previously obtained for a bipolar
configuration. Some differences between the two cases are however
observed: (1) the presence of an X-point above the twisted flux
rope makes the expulsion of the latter much easier due to the weaker
confinement near this point; this difference may be at the origin of
the existence of two classes of CMEs-fast and slow; (2) the energy W(t)
of the configuration remains smaller than the energy Wσ(t)
of the associated totally open field, and then the disruption does not
occur when W(t)~Wσ(t), as in the bipolar case. Rather we
get nonequilibrium when W(t)~WSO(t), where WSO(t)
is the energy of a semiopen field which has its open lines connected
to the two central spots on which flux cancellation is imposed. A
consequence of our results is that the topological complexity of a
preerupting configuration cannot be taken as a criterion for eliminating
the flux cancellation model in favor of the well-known breakout model.
Title: MHD Simulations of Small Active Regions Interacting with
Coronal Holes
Authors: Lionello, R.; Linker, J. A.; Mikić, Z.; Riley, P.
Bibcode: 2007AGUFMSH22A0845L
Altcode:
We use our 3D MHD algorithm in spherical coordinates to study the
interaction of the magnetic field of two small bipoles with that of a
coronal hole. To prescribe the magnetic flux distribution at the lower
boundary, we add two bipolar active regions to a smoothed Kitt Peak
magnetogram for Carrington Rotation 1913 (late August 1996). Starting
from a potential field extrapolation and a 1D solar wind solution,
we relax the configuration until coronal holes, streamers, and the
heliospheric current sheet are formed. At this point we introduce
surface flows, which evolve the magnetic flux distribution at the
boundary. We study the reconnection events and the changes in the
distribution of closed and open magnetic flux associated with the
bipolar active regions.
Title: Structure and Dynamics of the Sun's Open Magnetic Field
Authors: Antiochos, S. K.; DeVore, C. R.; Karpen, J. T.; Mikić, Z.
Bibcode: 2007ApJ...671..936A
Altcode: 2007arXiv0705.4430A
The solar magnetic field is the primary agent that drives solar
activity and couples the Sun to the heliosphere. Although the details
of this coupling depend on the quantitative properties of the field,
many important aspects of the corona-solar wind connection can be
understood by considering only the general topological properties of
those regions on the Sun where the field extends from the photosphere
out to interplanetary space, the so-called open field regions that are
usually observed as coronal holes. From the simple assumptions that
underlie the standard quasi-steady corona-wind theoretical models, and
that are likely to hold for the Sun as well, we derive two conjectures
as to the possible structure and dynamics of coronal holes: (1) coronal
holes are unique in that every unipolar region on the photosphere can
contain at most one coronal hole, and (2) coronal holes of nested
polarity regions must themselves be nested. Magnetic reconnection
plays the central role in enforcing these constraints on the field
topology. From these conjectures we derive additional properties for
the topology of open field regions, and propose several observational
predictions for both the slowly varying and transient corona/solar wind.
Title: Comparing Coronal Heating Models by Using Their Implied EUV
and Soft X-ray Emissions
Authors: Mok, Y.; Lionello, R.; Mikic, Z.; Linker, J. A.
Bibcode: 2007AGUFMSH21A0294M
Altcode:
The plasma heating mechanism that maintains the coronal temperature
remains poorly understood after decades of research. There have been
numerous theoretical models, but none of them has been confirmed by
observations. Each model has a different parametric dependence on
physical quantities, such as the local magnetic field, plasma density,
etc. Due to these differences, they imply different thermal structures
in the solar atmosphere, leading to different characteristics in
electromagnetic emissions. In this study, we examine these heating
models by comparing their predicted extreme ultraviolet (EUV) and soft
X-ray emissions with available observations. We use an active region
as a testing ground, partly because of its brightness, and partly
because its complex magnetic field can reveal the unique features of
each model. From the heat source, we compute the thermal structure in
the neighborhood of the active region in 3D for each model. A synthetic
emission image is then computed and compared with observations.
Title: Compression of the current sheet and its impact into the
reconnection rate
Authors: Vainshtein, S. I.; Mikić, Z.; Sagdeev, R.
Bibcode: 2007arXiv0711.1666V
Altcode:
Numerical simulations of strongly compressible MHD corresponding to
a stellar atmosphere with substantial gravity and near force-free
magnetic fields show that the current sheet collapses (its width
decreasing substantially). As a result, the reconnection rate increases
dramatically.
Title: Can Steady Heating Proportional to Magnetic Field Strength
Solve the Coronal Heating Paradox?
Authors: Winebarger, Amy R.; Mikic, Z.
Bibcode: 2007AAS...210.9123W
Altcode: 2007BAAS...39..208W
The time scale of the heating in solar corona has been studied through
comparisons of observations with the results of one-dimensional
hydrodynamic models for the past 30 years. Over this time,
two apparently contradictory conclusions have been reached. Some
observations, particularly of the hotter, shorter loops, are consistent
with steady heating while other observations, particularly of longer,
cooler loops, show that the loops are evolving and consistent with
impulsive heating. In this poster, we investigate a possible solution
to this paradox that has been heretofore overlooked, namely heating
that is steady and proportional to the local magnetic field strength
to some power. Short loops with near constant magnetic field along
their lengths will have solutions consistent with steady heating. Long
loops with large gradients in the magnetic field along their lengths
will have highly stratified heating. Such heating has no steady state
and hence produces dynamic solutions. We will discuss the properties
of evolving loops heated with highly stratified heating functions,
and we will compare them with X-ray and EUV observations.
Title: 3d Simulation Of Time Dependent Emissions From Active-region
Coronal Loops*
Authors: Mok, Yung; Mikic, Z.; Lionello, R.; Linker, J.
Bibcode: 2007AAS...210.9125M
Altcode: 2007BAAS...39Q.208M
We investigate the coronal heating mechanism by utilizing the EUV
and soft X-ray emissions from plasma loops in active regions. These
radiation signatures are believed to be strongly related to the thermal
structure of the loops, and therefore can be utilized as diagnostics
for the heating mechanism. The very existence of these emitting loops
and their formation severely constrain the heating models; namely, an
arbitrary heating model may not support a plasma loop in 3D dynamically
and thermally with properties consistent with observations. We have
identified a class of heating models that can lead to the formation of
these loops by solving a modified system of MHD equation supplemented
by an energy equation to take into account the heating, radiative
cooling and thermal conduction. Additional constraints are imposed
by the high-resolution observations from TRACE. Although the loops
observed by SXT/Yohkoh (soft X-ray) and EIT/SOHO (EUV) appear to be in
a quasi-steady state, data from TRACE (EUV) indicate that the brightness
of some loops is time dependent and plasma flows can be present. We scan
the parameter space of these heating models and further narrow down the
range to those that can reproduce the time dependent emissions which
mimic the TRACE images. *Work supported by Sun-Earth Connection
Theory Program of NASA
Title: The Multispectral Emission of the Sun during August 1996
Authors: Lionello, Roberto; Linker, J. A.; Mikic, Z.
Bibcode: 2007AAS...210.9108L
Altcode: 2007BAAS...39..205L
We investigated the structure of the solar corona during August
1996 using our 3D MHD model that includes thermal conduction along the
magnetic field, radiation losses, and heating. We produced synthetic
emission images in the extreme ultraviolet and X-rays and quantitatively
compared the results from different heating models with observations.
Title: Predicting the Structure of the Solar Corona for the Total
Solar Eclipse of March 29, 2006
Authors: Mikić, Z.; Linker, J. A.; Lionello, R.; Riley, P.; Titov, V.
Bibcode: 2007ASPC..370..299M
Altcode:
We describe the use of a three-dimensional MHD model to predict the
structure of the corona prior to the total solar eclipse of March 29,
2006. The calculation uses the observed photospheric radial magnetic
field as a boundary condition. We use a new version of our model that
has an improved description of energy transport in the corona. The
model allows us to predict the emission of X-ray and EUV radiation
in the corona. We compare the predicted polarization brightness in
the corona with four observations of the eclipse from Greece, Egypt,
and Libya, and we demonstrate that the model accurately predicts the
large-scale structure of the corona. We also compare X-ray emission
from the model with GOES/SXI images.
Title: Thermodynamic MHD Modeling of Coronal Mass Ejections
Authors: Linker, Jon A.; Lionello, R.; Mikic, Z.; Riley, P.; Titov, V.
Bibcode: 2007AAS...210.5805L
Altcode: 2007BAAS...39..168L
Coronal mass ejections (CMEs) disrupt the large-scale coronal magnetic
field and propel plasma and magnetic flux outward into interplanetary
space. The most energetic CMEs typically originate from active regions
on the Sun. Accurately modeling active regions while also capturing
the entire corona requires MHD models that include energy transport
(radiative losses,anisotropic thermal conduction, and coronal heating)
in the transition region and solar corona. We refer to this as the
thermodynamic MHD model. The more accurate representation of energy
flow in the thermodynamic MHD model allows us to to compute simulated
EUV and X-ray emission as would be observed from spacecraft such as
SOHO, STEREO, and Hinode. With this approach, theorists no longer get
to argue what emission they think their favorite model's magnetic field
evolution implies; we can actually go compute the emission and compare
with observations. As an example, we show a simulation of the May 12,
1997 CME, and compare the simulated emission with observations from
the actual event of dimming regions, postflare loops, and reformation
of loops near the northern polar coronal hole. Work supported
by NASA, NSF and the Center for Integrated Space Weather Modeling
(an NSF Science and Technology Center).
Title: Understanding the relationship between photospheric magnetic
field observations and in situ observations of the interplanetary
magnetic field
Authors: Riley, P.; Mikic, Z.; Linker, J. A.
Bibcode: 2007AGUSMSH23C..02R
Altcode:
Understanding the Sun's open flux and its variability during the course
of the solar cycle is important for a number of reasons. For example,
recent claims that it has increased significantly over the last
century may have had significant space- and even terrestrial-weather
consequences. A key relationship in understanding this evolution lies
between the observed photospheric magnetic field and the open flux
measured in situ by spacecraft. Global potential field source surface
(PFSS) and MHD models can be used to address this relationship;
however, several issues make this a difficult task. First, there is
controversy about how to convert un-calibrated magnetogram measurements
at some solar observatories into radial magnetic fields (which are the
primary input into most numerical models). Second, it is not clear
what contribution coronal mass ejections (CMEs) and other transient
phenomena make to the observed open flux in interplanetary space. Third,
it is difficult to assess what errors the different models introduce. In
this study we investigate the relationship between solar observations of
the photospheric magnetic field and in situ measurements by addressing
each issue systematically. Here, we focus on deriving the best estimate
of the photospheric magnetic field by inter-calibrating data from a
number of solar observatories and assessing the sensitivity of the
models to these inputs.
Title: CME Initiation in Active Regions
Authors: Mikic, Zoran; Linker, J. A.; Lionello, R.; Titov, V. S.;
Riley, P.
Bibcode: 2007AAS...210.2916M
Altcode: 2007BAAS...39..140M
We describe an idealized model of CME initiation in active regions based
on the mechanism of flux cancellation. This problem is characterized
by the coupling of local physics (on the scale of active regions and
smaller length scales) with global structures (on the solar radius
scale). A model of this kind may be needed to explain the origin of
fast CMEs. We will address the relationship between the photospheric
shearing flows and flux cancellation mechanism that are used to
energize the magnetic field and the characteristics of the pre- and
post-eruptive magnetic field. Research supported by NASA and
the Center for Integrated Space Weather Modeling (an NSF Science and
Technology Center).
Title: Understanding The Relationship Between Photospheric Magnetic
Field Observations And In Situ Observations Of The Interplanetary
Magnetic Field
Authors: Riley, Pete; Mikic, Z.; Linker, J. A.
Bibcode: 2007AAS...21010004R
Altcode: 2007BAAS...39..228R
Understanding the Sun's open flux and its variability during the course
of the solar cycle is important for a number of reasons. For example,
recent claims that it has increased significantly over the last
century may have had significant space- and even terrestrial-weather
consequences. A key relationship in understanding this evolution lies
between the observed photospheric magnetic field and the open flux
measured in situ by spacecraft. Global potential field source surface
(PFSS) and MHD models can be used to address this relationship;
however, several issues make this a difficult task. First, there is
controversy about how to convert un-calibrated magnetogram measurements
at some solar observatories into radial magnetic fields (which are the
primary input into most numerical models). Second, it is not clear
what contribution coronal mass ejections (CMEs) and other transient
phenomena make to the observed open flux in interplanetary space. Third,
it is difficult to assess what errors the different models introduce. In
this study we investigate the relationship between solar observations of
the photospheric magnetic field and in situ measurements by addressing
each issue systematically. Here, we focus on deriving the best estimate
of the photospheric magnetic field by inter-calibrating data from a
number of solar observatories and assessing the sensitivity of the
models to these inputs.
Title: May 12 1997 CME Event: A Simplified Model of the Pre-Eruptive
Magnetic Structure
Authors: Titov, Viacheslav; Mikic, Z.; Linker, J. A.; Lionello, R.
Bibcode: 2007AAS...210.2918T
Altcode: 2007BAAS...39..140T
A simple model of the coronal magnetic field prior to the CME
eruption on May 12 1997 is developed. First, this field is
constructed by superimposing a large-scale background field and
a localized bipolar field to model the active region (AR) in the
potential approximation. The background field is determined from
the observed photospheric normal field averaged over the longitude
of the Sun. The AR field is modeled by a subphotospheric dipole
whose parameters are optimized to fit the magnetic field obtained
from an MDI magnetogram. Second, this potential configuration is
quasi-statically sheared by photospheric vortex motions applied to two
flux concentrations of the AR. Third, the resulting force-free field
is then evolved by canceling the photospheric flux with the help of
a flow converging to the neutral line of the AR. To understand the
structure of the modeled configuration, we use a field line mapping
technique generalized to spherical geometry. It is demonstrated that the
initial configuration contains a hyperbolic flux tube (HFT) consisting
of two intersecting quasi-separatrix layers. This HFT provides a
partition of the closed magnetic flux between the AR and global solar
magnetic field. Such a partition is approximate since the entire flux
distribution is perfectly continuous. The vortex motions applied to
the AR interlock the coronal field lines to form additionally two new
HFTs pinched into thin current layers (CLs). These CLs carry the return
current shielding the twisted field of the AR from a nearly potential
background field. Reconnection in these CLs helps redistribute the flux
in the configuration during the cancellation phase. At this phase,
a magnetic flux rope is formed together with a bald patch separatrix
surface wrapping around the rope. Other important implications of
the identified structural features of the modeled configuration are
also discussed.
Title: ``Bursty'' Reconnection Following Solar Eruptions: MHD
Simulations and Comparison with Observations
Authors: Riley, Pete; Lionello, Roberto; Mikić, Zoran; Linker, Jon;
Clark, Eric; Lin, Jun; Ko, Yuan-Kuen
Bibcode: 2007ApJ...655..591R
Altcode:
Posteruptive arcades are frequently seen in the aftermath of coronal
mass ejections (CMEs). The formation of these loops at successively
higher altitudes, coupled with the classic ``two-ribbon'' flare seen
in Hα, are interpreted as reconnection of the coronal magnetic field
that has been dragged outward by the CME. White-light observations of
``rays,'' which have been interpreted as being coincident with the
current sheet at the reconnection site underneath the erupting CME,
also provide evidence for its occurrence. ``Blobs'' occasionally
seen within these rays suggest an even richer level of structure. In
this report, we present numerical simulations that reproduce both the
observed rays and the formation and evolution of the blobs. We compare
their properties with SOHO/LASCO observations of similar structures,
and relate their formation to standard theories of reconnection.
Title: MHD Modeling of Active Regions with Realistic Energy Transport*
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.; Titov, V.
Bibcode: 2006AGUFMSH33B0414L
Altcode:
Coronal mass ejections (CMEs) disrupt the large-scale coronal
magnetic field and propel plasma and magnetic flux outward into
interplanetary space. The fastest and most energetic CMEs typically
originate from active regions on the Sun. For active region CMEs both
the local active region magnetic field and the global magnetic fields
due to the surrounding magnetic flux are important. Modeling active
region magnetic fields presents special challenges, due to the range
of scales involved and the strong magnetic fields present in active
regions. This regime requires more accurate coronal models that include
energy transport (radiative losses, anisotropic thermal conduction,
and coronal heating) in the transition region. In this talk we describe
MHD models of active region fields embedded in the global corona, and
we discuss their eruptive properties. We discuss the implications of
our work for the initiation of fast CMEs. *Work supported by NASA,
NSF and the Center for Integrated Space Weather Modeling (an NSF
Science and Technology Center).
Title: A Comparison between Global Solar Magnetohydrodynamic and
Potential Field Source Surface Model Results
Authors: Riley, Pete; Linker, J. A.; Mikić, Z.; Lionello, R.; Ledvina,
S. A.; Luhmann, J. G.
Bibcode: 2006ApJ...653.1510R
Altcode:
The large-scale, steady-state magnetic field configuration of the
solar corona is typically computed using boundary conditions derived
from photospheric observations. Two approaches are typically used:
(1) potential field source surface (PFSS) models, and (2) the
magnetohydrodynamic (MHD) models. The former have the advantage that
they are simple to develop and implement, require relatively modest
computer resources, and can resolve structure on scales beyond those
that can be handled by current MHD models. However, they have been
criticized because their basic assumptions are seldom met. Moreover,
PFSS models cannot directly incorporate time-dependent phenomena,
such as magnetic reconnection, and do not include plasma or its
effects. In this study, we assess how well PFSS models can reproduce
the large-scale magnetic structure of the corona by making detailed
comparisons with MHD solutions at different phases in the solar activity
cycle. In particular, we (1) compute the shape of the source surface as
inferred from the MHD solutions to assess deviations from sphericity,
(2) compare the coronal hole boundaries as determined from the two
models, and (3) estimate the effects of nonpotentiality. Our results
demonstrate that PFSS solutions often closely match MHD results for
configurations based on untwisted coronal fields (i.e., when driven by
line-of-sight magnetograms). It remains an open question whether MHD
solutions will differ more substantially from PFSS solutions when vector
magnetograms are used as boundary conditions. This will be addressed
in the near future when vector data from SOLIS, the Solar Dynamics
Observatory, and Solar-B become incorporated into the MHD models.
Title: Maximizing the Scientific Return of the Sentinels Mission
using Global MHD Models
Authors: Riley, P.; Linker, J. A.; Mikic, Z.; Lionello, R.
Bibcode: 2006AGUFMSH53C..04R
Altcode:
The Sentinels mission promises to provide a unique view of the
acceleration and transport processes of energetic particles as well as
the initiation and evolution of coronal mass ejections (CMEs). A crucial
component in understanding the physics associated with these processes
lies in the large-scale structure of the corona and heliosphere,
particularly during the eruption and propagation of fast CMEs. In
this talk we review the current status of our MHD modeling efforts,
focusing on Sentinels-specific science, and project forward to envisage
what capabilities we may have developed by the time that the Sentinels
launch (2012). In conjunction with Solar Orbiter, the farside and
near-Earth Sentinels spacecraft will provide simultaneous photospheric
magnetograph measurements at multiple longitudes, which will lead to
major improvements in our ability to prescribe accurate, time-dependent
global boundary conditions. Data returned from the inner heliospheric
Sentinels will be used to validate these results. The model results can
be used in a variety of ways to interpret the observations. For example,
products from the model, such as the properties of CME- driven shocks
and CME-associated reconnection sites can be used to interpret complex
energetic particle profiles. Also, by tracing along magnetic field
lines, the inferred sites of the energetic particles can be connected
directly with the in situ measurements at each spacecraft. Perhaps more
so than any previous mission, sophisticated models will be required
to unravel the broad and disparate measurements returned by the suite
of Sentinels spacecraft.
Title: MHD Modeling of the Solar Corona through Differential Rotation
of the Magnetic Flux
Authors: Lionello, R.; Riley, P.; Linker, J. A.; Mikic, Z.
Bibcode: 2006AGUFMSH51A1456L
Altcode:
We model the evolution of the coronal magnetic field for several solar
rotation using our 3D MHD algorithm in spherical coordinates. After
imposing an initial magnetic flux distribution extracted from a
magnetogram, we relax the system to steady state. We then evolve the
magnetic flux distribution through differential rotation. The response
of the coronal and heliospheric magnetic field is analyzed.
Title: Goals and Progress of the LWS Focused Science Topic on the
CME--ICME Connection
Authors: Mikic, Z.; Deforest, C.; Devore, R.; Georgoulis, M.; Jackson,
B.; Nitta, N.; Pizzo, V.; Odstrcil, D.
Bibcode: 2006AGUFMSH21B..05M
Altcode:
Our team addresses the NASA Living With a Star (LWS) Focused Science
Topic "to determine the solar origins of the plasma and magnetic flux
observed in an interplanetary Coronal Mass Ejection (ICME)." In short,
this team is examining the CME--ICME connection. Our team was formed
as a result of awards from the LWS Targeted Research &Technology
competition in the fall of 2004. Our team is investigating the detailed
relationship between the plasma and magnetic fields in active regions,
the source regions of CMEs, and subsequent in situ measurements in
interplanetary magnetic clouds. We plan to study this connection through
detailed numerical simulations of CME initiation and propagation,
theoretical investigations, and studies of the properties of active
regions, CMEs, and magnetic clouds. We will discuss the goals of
our team, how it fits into NASA's missions, and our progress so
far. Research supported by NASA's Living With a Star Program.
Title: Modelling Active Region Magnetic Fields in the Context of
the Global Corona
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Titov, V. S.;
Riley, P.
Bibcode: 2006IAUJD...3E..61L
Altcode:
Coronal mass ejections (CMEs) disrupt the large-scale coronal magnetic
field and propel plasma and magnetic flux outward into interplanetary
space. The fastest and most energetic CMEs typically originate from
active regions on the Sun. For active region CMEs both the local
active region magnetic field and the global magnetic fields due
to the surrounding magnetic flux are important. Modelling active
region magnetic fields in the context of the global corona presents
special challenges, due to the range of scales involved and the strong
magnetic fields present in active regions. This regime requires more
accurate coronal models that include energy transport (radiative
losses, anisotropic thermal conduction, and coronal heating) in the
transition region. In this talk we describe MHD models of active region
fields embedded in the global corona, and we discuss their eruptive
properties. We discuss the implications of our work for the initiation
of fast CMEs. Work supported by NASA, NSF and the Center for Integrated
Space Weather Modeling (an NSF Science and Technology Center).
Title: Slow Coronal Wind Composition
Authors: Abbo, Lucia; Antonucci, Ester; Dodero, Maria Adele; Mikić,
Zoran; Riley, Pete
Bibcode: 2006ESASP.617E..17A
Altcode: 2006soho...17E..17A
No abstract at ADS
Title: The Evolution of the Magnetic Field Structure for the May 12,
1997 CME Event*
Authors: Titov, Viacheslav; Mikic, Z.; Linker, J. A.; Lionello, R.;
Riley, P.
Bibcode: 2006SPD....37.0901T
Altcode: 2006BAAS...38..235T
We analyze the evolving magnetic field structure for the May
12,1997 CME event by using a three-step numerical approach. First,
thecoronal magnetic field prior to the CME eruption on May 12,1997 is
extrapolated in the potential approximation. Second,this configuration
is then sheared and twistedquasi-statically to build up a force-free
configurationwith a free magnetic energy sufficient for eruption. At
thethird step, this configuration is driven to eruption bycanceling
the twisted magnetic flux at the polarityinversion line. We demonstrate
that the configurationcontains from the beginning a so-called hyperbolic
fluxtube which is a union of two intersecting quasi-separatrixlayers. At
later stages, other structural featuresdevelop in the evolving
configuration. They are analyzedand related to the characteristics of
the observed eruption.*Research supported by NASA and the Center for
IntegratedSpace Weather Modeling (an NSF Science and TechnologyCenter).
Title: Modeling the Global Solar Corona with Improved Energy Transport
Authors: Mikic, Zoran; Linker, J. A.; Lionello, R.; Riley, P.; Titov,
V.; Mok, Y.
Bibcode: 2006SPD....37.1405M
Altcode: 2006BAAS...38..244M
We describe the application of a global three-dimensional
magnetohydrodynamic (MHD) model of the solar corona that includes
improved energy transport. The energy equation includes parameterized
coronal heating, thermal conduction parallel to the magnetic field,
radiative losses, and the acceleration due to Alfvén waves. This
model makes it possible to determine the large-scale structure of the
magnetic field in the corona, as well as the distribution of the solar
wind velocity, plasma density, and temperature. The calculation uses the
observed photospheric radial magnetic field as a boundary condition. We
will describe the use of the model to predict the structure of the
solar corona prior to the total solar eclipse that occurred on March
29, 2006. We used magnetic fields observed on the solar disk prior
to eclipse day to predict what the corona will look like during the
eclipse. The estimated coronal density and temperature was used to
predict the plane-of-sky polarization brightness and emission of
EUV and X-ray radiation. The prediction was posted on our web site
(http://iMHD.net) prior to the eclipse. We will compare the prediction
with eclipse observations..Supported by NASA's Sun-Earth Connection
Theory and SR&T Programs and by NSF's CISM project
Title: MHD Modeling of Coronal Mass Ejections: A "Simple" Event
Authors: Linker, Jon A.; Mikic, Z.; Lionello, R.; Titov, V.; Riley, P.
Bibcode: 2006SPD....37.2202L
Altcode: 2006BAAS...38Q.249L
In previous work we demonstrated a fast ( 1000 km/s) eruption in an
idealized model of the Sun's magnetic field around the time of the
May 12, 1997 CME. The model, motivated by an MDI synoptic magnetic
field map, contained the sum of a localized bipole (representing an
active region) and a large scale axisymmetric magnetic field of dipolar
character. Using a zero-beta MHD model, we studied the energization of
this magnetic field by shearing flows in the photosphere, as well as
an eruption initiated by flux cancellation. While this simple model
captured the essence of the magnetic field structure, it limited our
ability to compare with CME observations. We will present an improved
model that has a more realistic magnetic field distribution, and we will
consider the effect of the solar wind on the magnetic field structure
and eruption.Research supported by NASA and the Center for Integrated
Space Weather Modeling (an NSF Science and Technology Center).
Title: 3D Simulations of Dynamic Coronal Loops
Authors: Mok, Yung; Mikic, Z.; Lionello, R.; Linker, J. A.
Bibcode: 2006SPD....37.1704M
Altcode: 2006BAAS...38Q.246M
Active-region coronal loops have been observed in EUV by EIT/SOHO and
TRACE and in soft X-ray by SXT/Yohkoh for many years. Although the loops
observed by EIT and Yohkoh appear to be in a quasi-steady state, TRACE
is able to capture their dynamic nature at high resolution, showing
their brightening and fading in time. The activity, as well as the
characteristics of the emissions, is believed to be strongly related
to plasma heating, the major unknown factor in the energy equation
and an outstanding problem in solar physics. We have constructed
the thermal structure of an active region in 3D for several heating
models. One of them results in a highly dynamic structure without
settling into a quasi-steady state. Each flux tube has its own time
variation characteristics in temperature and density. We then computed
the time dependent EUV emissions of the entire region, and found that it
appears to closely resemble the images seen in the TRACE movies; namely,
brightening and fading of thin, EUV emitting plasma loops of nearly
uniform cross section. Quantitative comparison between the results of
our simulation and EIT/SXT measurements will be made.*This research
is supported by the Sun-Earth Connection Theory Program of NASA.
Title: On the rates of coronal mass ejections: remote solar and in
situ observations
Authors: Riley, P.; Cane, H.; Richardson, I. G.; Gopalswamy, N.;
Linker, J. A.; Mikic, Z.; Lionello, R.
Bibcode: 2006AGUSMSA21A..01R
Altcode:
In this study we compare the rates of coronal mass ejections (CMEs)
as inferred from remote solar observations and interplanetary CMEs
(ICMEs) as inferred from in situ observations at both 1 AU and Ulysses
for almost an entire solar cycle (1996 through 2004). We find that,
while the rates of CMEs and ICMEs track each other well at solar
minimum, they diverge significantly in early 1998, during the ascending
phase of the solar cycle, with the remote solar observations yielding
approximately 20 times more events than are seen in situ at 1 AU. This
divergence persists through 2004. We discuss several possible causes,
including: (1) the appearance of mid-latitude active regions; (2)
the increased rate of high-latitude CMEs; and (3) the strength of the
global solar field. We conclude that the most likely interpretation
is that this divergence is due to the birth of mid-latitude active
regions, which are the sites of a distinct population of CMEs that are
only partially intercepted by Earth. This conclusion is supported by
the following points: (1) A similar divergence occurs between ICMEs in
which magnetic clouds are observed (MCs), and those that are not; and
(2) a number of pronounced enhancements in the CME rate, separated
by approximately one year, are also mirrored and in ICME rate, but
not obviously in the MC rate. We provide a simple geometric argument
that shows that the computed CME and ICME rates are consistent with
each other. The origins of the individual peaks can be traced back
to unusually strong active regions on the Sun. Taken together, these
results suggest that whether one observes a flux rope within an ICME
is sensitive to the trajectory of the spacecraft through the ICME,
i.e., an observational selection effect. This conclusion is supported
by models of CME eruption and evolution, which: (1) are incapable of
producing a CME that does not contain an embedded flux rope; and (2)
demonstrate that glancing intercepts can produce ICME-like signatures
without the magnetic structures associated with a flux rope
Title: Global MHD Models of the Solar Corona with Realistic Energy
Transport*
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.
Bibcode: 2006AGUSMSH52A..05L
Altcode:
Global MHD models of the corona and solar wind that use boundary
conditions based on observed photospheric magnetic fields have now
existed for more than a decade. Typically these models have avoided
the complicated physics of the transition region by setting the
ratio of specific heats (γ) to a reduced value. These "polytropic"
MHD models have been successful in describing a number of aspects
of coronal and heliospheric data, including the location of coronal
holes, the reproduction of streamer structure as seen in white light,
and the location of the heliospheric current sheet. The polytropic
approach also has fundamental limitations, for example density and
temperature contrasts between open and closed field regions do not
match observations, and the model does not address data from EUV and
X-ray emission. In this paper, we describe MHD models of the global
corona that include energy transport (radiative losses, anisotropic
thermal conduction, and coronal heating) in the transition region and
solar corona and are capable of reproducing many emission properties
as observed by SOHO and Yohkoh. These calculations are made tractable
by a significant advance we have made in our ability to efficiently
model the coupling between the transition region and corona. With this
advance, it should be possible to routinely model the global corona,
as is now possible with polytropic models. *Work supported by NASA,
NSF and the Center for Integrated Space Weather Modeling (an NSF
Science and Technology Center).
Title: The Latitudinal Excursion of Coronal Magnetic Field Lines in
Response to Differential Rotation: MHD Simulations
Authors: Lionello, Roberto; Linker, Jon A.; Mikić, Zoran; Riley, Pete
Bibcode: 2006ApJ...642L..69L
Altcode:
Solar energetic particles, which are believed to originate from
corotating interacting regions (CIRs) at low heliographic latitude,
were observed by the Ulysses spacecraft even as it passed over the
Sun's poles. One interpretation of this result is that high-latitude
field lines intercepted by Ulysses connect to low-latitude CIRs
at much larger heliocentric distances. The Fisk model explains the
latitudinal excursion of magnetic field lines in the solar corona
and heliosphere as the inevitable consequence of the interaction of a
tilted dipole in a differentially rotating photosphere with rigidly
rotating coronal holes. We use a time-dependent three-dimensional
magnetohydrodynamic (MHD) algorithm to follow the evolution of a
simple model of the solar corona in response to the differential
rotation of the photospheric magnetic flux. We examine the changes of
the coronal-hole boundaries, the redistribution of the line-of-sight
magnetic field, and the precession of field lines in the corona. Our
results confirm the basic idea of the Fisk model, that differential
rotation leads to changes in the heliographic latitude of magnetic
field lines. However, the latitudinal excursion of magnetic field
lines in this simple ``tilted dipole'' model is too small to explain
the Ulysses observations. Although coronal holes in our model rotate
more rigidly than do photospheric features (in general agreement
with observations), they do not rotate strictly rigidly as assumed
by Fisk. This basic difference between our model and Fisk's will be
explored in the future by considering more realistic magnetic flux
distributions, as observed during Ulysses polar excursions.
Title: The Pre-CME Sun
Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades,
H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M.
Bibcode: 2006SSRv..123..303G
Altcode: 2006SSRv..tmp...77G
The coronal mass ejection (CME) phenomenon occurs in closed magnetic
field regions on the Sun such as active regions, filament regions,
transequatorial interconnection regions, and complexes involving a
combination of these. This chapter describes the current knowledge
on these closed field structures and how they lead to CMEs. After
describing the specific magnetic structures observed in the CME source
region, we compare the substructures of CMEs to what is observed before
eruption. Evolution of the closed magnetic structures in response to
various photospheric motions over different time scales (convection,
differential rotation, meridional circulation) somehow leads to the
eruption. We describe this pre-eruption evolution and attempt to link
them to the observed features of CMEs. Small-scale energetic signatures
in the form of electron acceleration (signified by nonthermal radio
bursts at metric wavelengths) and plasma heating (observed as compact
soft X-ray brightening) may be indicative of impending CMEs. We survey
these pre-eruptive energy releases using observations taken before
and during the eruption of several CMEs. Finally, we discuss how the
observations can be converted into useful inputs to numerical models
that can describe the CME initiation.
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena.
Report of Working Group F
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
Bibcode: 2006SSRv..123..341P
Altcode: 2006SSRv..tmp...60P
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since CMEs
originate in the low solar corona, understanding the physical processes
that generate them is strongly dependant on coordinated multi-wavelength
observations. CMEs display a large diversity in morphology and kinematic
properties, but there is presently no statistical evidence that those
properties may serve to group them into different classes. When a CME
takes place, the coronal magnetic field undergoes restructuring. Much
of the current research is focused on understanding how the corona
sustains the stresses that allow the magnetic energy to build up and
how, later on, this magnetic energy is released during eruptive flares
and CMEs. Multi-wavelength observations have confirmed that reconnection
plays a key role during the development of CMEs. Frequently, CMEs
display a rather simple shape, exhibiting a well known three-part
structure (bright leading edge, dark cavity and bright knot). These
types of events have led to the proposal of the ‘`standard model’'
of the development of a CME, a model which predicts the formation
of current sheets. A few recent coronal observations provide some
evidence for such sheets. Other more complex events correspond to
multiple eruptions taking place on a time scale much shorter than the
cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. “Complex type III-like events,”which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.
Title: An Introduction to Theory and Models of CMEs, Shocks, and
Solar Energetic Particles
Authors: Mikić, Z.; Lee, M. A.
Bibcode: 2006SSRv..123...57M
Altcode: 2006SSRv..tmp...64M
We present a brief introduction to the essential physics of coronal
mass ejections as well as a review of theory and models of CME
initiation, solar energetic particle (SEP) acceleration, and shock
propagation. A brief review of the history of CME models demonstrates
steady progress toward an understanding of CME initiation, but it is
clear that the question of what initiates CMEs has still not been
solved. For illustration, we focus on the flux cancellation model
and the breakout model. We contrast the similarities and differences
between these models, and we examine how their essential features
compare with observations. We review the generation of shocks by
CMEs. We also outline the theoretical ideas behind the origin of a
gradual SEP event at the evolving CME-driven coronal/interplanetary
shock and the origin of “impulsive” SEP events at flare sites of
magnetic reconnection below CMEs. We argue that future developments in
models require focused study of “campaign events” to best utilize
the wealth of available CME and SEP observations.
Title: CME Theory and Models
Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.;
Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.;
Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P.
Bibcode: 2006SSRv..123..251F
Altcode: 2006SSRv..tmp...59F
This chapter provides an overview of current efforts in the theory and
modeling of CMEs. Five key areas are discussed: (1) CME initiation;
(2) CME evolution and propagation; (3) the structure of interplanetary
CMEs derived from flux rope modeling; (4) CME shock formation in the
inner corona; and (5) particle acceleration and transport at CME driven
shocks. In the section on CME initiation three contemporary models are
highlighted. Two of these focus on how energy stored in the coronal
magnetic field can be released violently to drive CMEs. The third
model assumes that CMEs can be directly driven by currents from below
the photosphere. CMEs evolve considerably as they expand from the
magnetically dominated lower corona into the advectively dominated
solar wind. The section on evolution and propagation presents two
approaches to the problem. One is primarily analytical and focuses on
the key physical processes involved. The other is primarily numerical
and illustrates the complexity of possible interactions between the
CME and the ambient medium. The section on flux rope fitting reviews
the accuracy and reliability of various methods. The section on shock
formation considers the effect of the rapid decrease in the magnetic
field and plasma density with height. Finally, in the section on
particle acceleration and transport, some recent developments in
the theory of diffusive particle acceleration at CME shocks are
discussed. These include efforts to combine self-consistently the
process of particle acceleration in the vicinity of the shock with
the subsequent escape and transport of particles to distant regions.
Title: Modeling interplanetary coronal mass ejections
Authors: Riley, Pete; Linker, J. A.; Mikic, Z.; Odstrcil, Dusan
Bibcode: 2006AdSpR..38..535R
Altcode:
Heliospheric models of Coronal Mass Ejection (CME) propagation and
evolution provide an important insight into the dynamics of CMEs
and are a valuable tool for interpreting interplanetary in situ
observations. Moreover, they represent a virtual laboratory for
exploring conditions and regions of space that are not conveniently
or currently accessible by spacecraft. In this report, we summarize
our recent advances in modeling the properties and evolution of
CMEs in the solar wind. We describe our current state of research
with three examples: (1) interpreting the global context of in situ
observations; (2) identifying new phenomena in the simulations; and
(3) differentiating between CME initiation models. We conclude by
discussing what topics will likely be important for models to address
in the future.
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
Bibcode: 2006cme..book..341P
Altcode:
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since
CMEs originate in the low solar corona, understanding the physical
processes that generate them is strongly dependant on coordinated
multi-wavelength observations. CMEs display a large diversity in
morphology and kinematic properties, but there is presently no
statistical evidence that those properties may serve to group them
into different classes. When a CME takes place, the coronal magnetic
field undergoes restructuring. Much of the current research is focused
on understanding how the corona sustains the stresses that allow the
magnetic energy to build up and how, later on, this magnetic energy is
released during eruptive flares and CMEs. Multiwavelength observations
have confirmed that reconnection plays a key role during the development
of CMEs. Frequently, CMEs display a rather simple shape, exhibiting a
well known three-part structure (bright leading edge, dark cavity and
bright knot). These types of events have led to the proposal of the
"standard model" of the development of a CME, a model which predicts
the formation current sheets. A few recent coronal observations provide
some evidence for such sheets. Other more complex events correspond
to multiple eruptions taking place on a time scale much shorter than
the cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. "Complex type III-like events," which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.
Title: An Introduction to Theory and Models of CMEs, Shocks, and
Solar Energetic Particles
Authors: Mikić, Z.; Lee, M. A.
Bibcode: 2006cme..book...57M
Altcode:
We present a brief introduction to the essential physics of coronal
mass ejections as well as a review of theory and models of CME
initiation, solar energetic particle (SEP) acceleration, and shock
propagation. A brief review of the history of CME models demonstrates
steady progress toward an understanding of CME initiation, but it is
clear that the question of what initiates CMEs has still not been
solved. For illustration, we focus on the flux cancellation model
and the breakout model. We contrast the similarities and differences
between these models, and we examine how their essential features
compare with observations. We review the generation of shocks by
CMEs. We also outline the theoretical ideas behind the origin of a
gradual SEP event at the evolving CME-driven coronal/interplanetary
shock and the origin of "impulsive" SEP events at flare sites of
magnetic reconnection below CMEs. We argue that future developments
in models require focused study of "campaign events" to best utilize
the wealth of available CME and SEP observations.
Title: CME Theory and Models
Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.;
Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.;
Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P.
Bibcode: 2006cme..book..251F
Altcode:
This chapter provides an overview of current efforts in the theory and
modeling of CMEs. Five key areas are discussed: (1) CME initiation;
(2) CME evolution and propagation; (3) the structure of interplanetary
CMEs derived from flux rope modeling; (4) CME shock formation in the
inner corona; and (5) particle acceleration and transport at CME driven
shocks. In the section on CME initiation three contemporary models are
highlighted. Two of these focus on how energy stored in the coronal
magnetic field can be released violently to drive CMEs. The third
model assumes that CMEs can be directly driven by currents from below
the photosphere. CMEs evolve considerably as they expand from the
magnetically dominated lower corona into the advectively dominated
solar wind. The section on evolution and propagation presents two
approaches to the problem. One is primarily analytical and focuses on
the key physical processes involved. The other is primarily numerical
and illustrates the complexity of possible interactions between the
CME and the ambient medium. The section on flux rope fitting reviews
the accuracy and reliability of various methods. The section on shock
formation considers the effect of the rapid decrease in the magnetic
field and plasma density with height. Finally, in the section on
particle acceleration and transport, some recent developments in
the theory of diffusive particle acceleration at CME shocks are
discussed. These include efforts to combine self-consistently the
process of particle acceleration in the vicinity of the shock with
the subsequent escape and transport of particles to distant regions.
Title: The Pre-CME Sun
Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades,
H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M.
Bibcode: 2006cme..book..303G
Altcode:
The coronal mass ejection (CME) phenomenon occurs in closed magnetic
field regions on the Sun such as active regions, filament regions,
transequatorial interconnection regions, and complexes involving a
combination of these. This chapter describes the current knowledge
on these closed field structures and how they lead to CMEs. After
describing the specific magnetic structures observed in the CME source
region, we compare the substructures of CMEs to what is observed before
eruption. Evolution of the closed magnetic structures in response to
various photospheric motions over different time scales (convection,
differential rotation, meridional circulation) somehow leads to the
eruption. We describe this pre-eruption evolution and attempt to link
them to the observed features of CMEs. Small-scale energetic signatures
in the form of electron acceleration (signified by nonthermal radio
bursts at metric wavelengths) and plasma heating (observed as compact
soft X-ray brightening) may be indicative of impending CMEs. We survey
these pre-eruptive energy releases using observations taken before
and during the eruption of several CMEs. Finally, we discuss how the
observations can be converted into useful inputs to numerical models
that can describe the CME initiation.
Title: Numerical Modeling of Solar Wind and Interplanetary CMEs
Authors: Odstrcil, D.; Arge, C. N.; Linker, J. A.; Liu, Y.; Mikic,
Z.; Pizzo, V. J.; Riley, P.; Zhao, X. P.
Bibcode: 2006cosp...36.3424O
Altcode: 2006cosp.meet.3424O
Numerical modeling plays a critical role in efforts to understand
the connection between solar eruptive phenomena and their impacts
in the near-Earth space environment and in interplanetary space The
Center for Integrated Space Weather Modeling CISM was established to
aid in developing models for space weather research and forecasting
We will show examples of existing capabilities of the heliospheric
modeling system driven by various coronal models and its application
to simulation of real events
Title: CISM space weather modeling of the Sun-to-Earth system
Authors: Wang, W.; Burns, A. G.; Hughes, W. J.; Linker, J. A.;
Luhmann, J. G.; Lyon, L. G.; Mikic, Z.; Odstrcil, D.; Solomon, S. C.;
Wiltberger, M.
Bibcode: 2006cosp...36.2569W
Altcode: 2006cosp.meet.2569W
The Center for Integrated Space weather Modeling CISM simulates the
entire Solar-terrestrial system by coupling several codes together
These codes range from the solar corona to the earth s upper atmosphere
and include the MAS solar corona model the ENIL solar wind model the
LFM magnetosphere code and the thermosphere ionosphere TING model
Several additional models such as the solar SEP model the RCM inner
magnetosphere model and a radiation belt model are also included
to provide additional information about space weather events The
CISM suite of models is coupled using Intercomm for interprocess
communication and Overture for grid interpolation This suite of models
has been successfully run to simulate a CME that propagates through the
heliosphere to affect the geospace environment and is being validated
and verified for observed events Visualization tools have also been
developed both to analyze model results and to facilitate the transition
of the CISM model to operational use
Title: The Impact of the Magnetic Field Structure on May 12, 1997
CME Event*
Authors: Titov, V. S.; Mikic, Z.; Linker, J. A.; Lionello, R.;
Riley, P.
Bibcode: 2005AGUFMSH13A0284T
Altcode:
We analyze the evolving magnetic field structure in May 12, 1997 CME
event by using a numerical model constructed in the following three
steps. First, an analytical model of the coronal magnetic field prior
to the CME eruption on May 12, 1997 is developed in the current-free
approximation. It is constructed by superimposing a large-scale
background field and a localized bipole field to model the active
region. The parameters of the model are optimized to fit the potential
field obtained from an MDI magnetogram. Second, this configuration
is then sheared and twisted in a quasi-static manner to build up a
force-free configuration with a free magnetic energy sufficient for
eruption. At the third step, this configuration is driven to eruption by
canceling the twisted magnetic flux at the polarity inversion line. We
demonstrate that the configuration contains from the beginning the
so-called hyperbolic flux tube which is a union of two intersecting
quasi-separatrix layers. At later stages, other structural features
are developing in the evolving configuration. They are analyzed and
related to the characteristics of the observed eruption. *Research
supported by NASA and the Center for Integrated Space Weather Modeling
(an NSF Science and Technology Center).
Title: Unraveling STEREO Observations with Global MHD Models
Authors: Riley, P.; Linker, J. A.; Mikic, Z.; Lionello, R.; Clark, E.
Bibcode: 2005AGUFMSH11B0261R
Altcode:
The STEREO (Solar TErrestrial RElations Observatory) mission will
employ two nearly identical spacecraft to study CME initiation and
evolution through the inner heliosphere and understand how energetic
particles are accelerated in the corona and solar wind. To accomplish
these goals, one spacecraft will drift ahead of Earth and the other will
trail behind, leading to a unique set of stereoscopic measurements of
the solar environment. In this study use global MHD simulations of CME
initiation and evolution to anticipate similarities and differences
between measurements that will be made by the two STEREO spacecraft
during the lifetime of the mission. In addition, we attempt to use
these same simulated observations to reconstruct the three-dimensional
properties of CMEs and the ambient solar wind. We assess to what extent
this procedure is successful by comparing the reconstructed results
with the original global simulation results. Finally, we demonstrate a
prototype GUI-driven tool that merges STEREO spacecraft trajectories
and viewpoints with our simulations to provide a global context for
interpreting STEREO observations.
Title: MHD Modeling of the Latitudinal Excursion of Magnetic Field
Lines in the Solar Corona
Authors: Lionello, R.; Riley, P.; Linker, J. A.; Mikić, Z.
Bibcode: 2005AGUFMSH11A0250L
Altcode:
In the Fisk model differential rotation on the solar photosphere is
thought to cause large excursions of magnetic field lines in the upper
corona and in the heliosphere. This is in contrast with the apparently
rigid rotation of some coronal holes. The model of Fisk et al. (1999)
consists of a dipole field with the magnetic axis distinct from the
rotation axis. The field from the northern and southern polar holes
superexpands into the solar wind. In order to reconcile the pattern of
velocity in the upper corona with the photospheric differential flow,
magnetic reconnection is invoked between open field lines and low-lying
loops. This mechanism releases the plasma trapped in the closed field
lines into the corona and originates the slow solar wind. We have used
our MHD model in spherical coordinates to study this mechanism. We have
imposed a magnetic flux distribution as in Fisk et al. (1999). After
relaxing the system to steady state, we have applied differential
rotation for the equivalent of 5 rotations. We will describe the changes
in the coronal magnetic field in respo nse to the photospheric flows.
Title: MHD Modeling of the May 12, 1997 CME
Authors: Linker, J. A.; Mikic, Z.; Titov, V.; Lionello, R.; Riley, P.
Bibcode: 2005AGUFMSH13A0283L
Altcode:
In previous work we constructed an idealized model of the coronal
magnetic field on May 11, 1997, preceding the CME that occurred on
the following day. The model, motivated by an MDI synoptic magnetic
field map, contained the sum of a localized bipole (representing an
active region) and a large scale axisymmetric magnetic field of dipolar
character. Using a zero-beta MHD model, we studied the energization of
this magnetic field by shearing flows in the photosphere, as well as
an eruption initiated by flux cancellation. While this simple model
captured the essence of the magnetic field structure, it limited our
ability to compare with CME observations. We will present an improved
model that has a more realistic magnetic field distribution, and we will
consider the effect of the solar wind on the magnetic field structure
and eruption. Research supported by NASA and the Center for Integrated
Space Weather Modeling (an NSF Science and Technology Center).
Title: Time-Dependent Response of the Large-Scale Solar Corona
Authors: Linker, J. A.; Lionello, R.; Mikic, Z.; Riley, P.
Bibcode: 2005ESASP.596E..28L
Altcode: 2005ccmf.confE..28L
No abstract at ADS
Title: How Good are Potential Field Source Surface Models? What the
MHD Modelers Don't Want you to Know
Authors: Riley, P.; Linker, J. A.; Mikic, Z.; Lionello, R.; Ledvina, S.
Bibcode: 2005AGUSMSH23C..02R
Altcode:
The large-scale, steady-state magnetic field configuration of the
solar corona is typically computed using boundary conditions derived
from photospheric observations. The two most popular approaches in use
today are: (1) potential field, source surface (PFSS) models; and (2)
magnetohydrodynamic (MHD) models. The former have the advantage that
they are: simple to develop and implement; require relatively modest
computer resources; and can resolve global structure on spatial scales
beyond those that can be handled by current MHD models. On the other
hand, they have been criticized because their basic assumptions (that
the field is potential and that a single, spherical source surface
exists) are seldom, if ever, met. In addition, PFSS models cannot
directly incorporate time dependent phenomena, such as magnetic
reconnection. In this study, we assess how well PFSS models can
reproduce the large-scale quasi-static magnetic structure of the
corona by making detailed comparisons with MHD solutions at different
phases in the solar activity cycle. Specifically, we: (1) compute
the shape of the source surface as inferred from the MHD solutions;
(2) compare the coronal hole boundaries as determined using the two
models; and (3) correlate the open flux determined from the models
with the magnetic flux observed at 1 AU. Our results suggest that PFSS
models compare relatively well with MHD computations of untwisted
coronal fields (matched to line-of-sight magnetograms). It remains
an open question how well PFSS models compare with MHD models that
match vector magnetograms. This question can be addressed once data
from SOLIS and Solar-B are incorporated into the MHD models.
Title: The Effects of Differential Rotation on the Magnetic Structure
of the Solar Corona: Magnetohydrodynamic Simulations
Authors: Lionello, Roberto; Riley, Pete; Linker, Jon A.; Mikić, Zoran
Bibcode: 2005ApJ...625..463L
Altcode:
Coronal holes are magnetically open regions from which the solar
wind streams. Magnetic reconnection has been invoked to reconcile
the apparently rigid rotation of coronal holes with the differential
rotation of magnetic flux in the photosphere. This mechanism might also
be relevant to the formation of the slow solar wind, the properties of
which seem to indicate an origin from the opening of closed magnetic
field lines. We have developed a global MHD model to study the effect
of differential rotation on the coronal magnetic field. Starting from
a magnetic flux distribution similar to that of Wang and coworkers,
which consists of a bipolar magnetic region added to a background
dipole field, we applied differential rotation over a period of 5 solar
rotations. The evolution of the magnetic field and of the boundaries of
coronal holes are in substantial agreement with the findings of Wang
and coworkers. We identified examples of interchange reconnection and
other changes of topology of the magnetic field. Possible consequences
for the origin of the slow solar wind are also discussed.
Title: The Evolution of MHD Modeling: Coupling Local and Global Scales
Authors: Mikic, Z.; Linker, J. A.; Riley, P.; Lionello, R.; Titov,
V. S.; Odstrcil, D.
Bibcode: 2005AGUSMSH11B..02M
Altcode:
We will describe how MHD models of the solar corona and inner
heliosphere have evolved over the years to include a more comprehensive
coupling of local physics (on the scale of active regions and smaller
length scales) with global structures (on the solar radius scale). In
particular, we will discuss our efforts to understand the thermal and
magnetic structure and dynamics of active regions and their coupling to
the global structure of the solar corona. We will discuss initiation
of CMEs that originate in active regions, and their propagation in
the inner heliosphere. Research supported by NASA and the Center
for Integrated Space Weather Modeling (an NSF Science and Technology
Center).
Title: Investigating the Coronal Heating Models at High Resolution
Authors: Mok, Y.; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2005AGUSMSP41A..02M
Altcode:
Active regions are an excellent testing ground for coronal heating
models because of their complex magnetic-field topology. Due to
their distinctive parametric dependences on the magnetic field and
plasma properties, each heating mechanism tends to deposit energy in
preferential locations. The thermal structure of the atmosphere is
further made distinctive by the local magnetic field through the highly
anisotropic thermal conductivity. As a result, each heating model gives
rise to unique radiation signatures, including EUV and soft X-ray. The
observed EUV and soft X-ray emissions can then be used to test the
validity of the models. Unfortunately, computing the thermal structure
in 3D encounters tremendous difficulty because of the extremely steep
gradients in temperature and density in the transition region, even
using a variable-size mesh. In our previous investigations, we were
forced to use an approximated thermal conductivity and compute the
structure at low resolution. We have developed a new, and improved,
method to treat the transition region so that the radiation signatures
are not affected by the approximations. Quantitative comparison with
observations becomes possible and will be presented. Using this highly
efficient method, we have also studied the coronal responses to time
dependent heating. Work supported by The Sun-Earth Connection Theory
Program of NASA
Title: Modeling Active Region Coronal Mass Ejections*
Authors: Linker, J. A.; Mikic, Z.; Titov, V.; Lionello, R.; Riley, P.
Bibcode: 2005AGUSMSH54B..05L
Altcode:
The fastest coronal mass ejections (CMEs) typically originate from
active regions on the Sun. From a theoretical standpoint, fast CMEs
are the most difficult to understand and model, because they require
that large amounts of magnetic energy (1032 ergs) be released rapidly
(~1000 seconds). We describe MHD computations of eruptive behavior
in an active region arising from the flux cancellation mechanism. The
active region is modeled as a localized bipole within a global dipolar
configuration, and is similar to the solar magnetic flux for the May
12, 1997 CME. We find that a localized 3D configuration erupts as a
consequence of flux cancellation, in the same way that 2D axisymmetric
and large-scale 3D fields erupt. We discuss the role of the interaction
of the local active region magnetic field with the global magnetic
fields due to the surrounding magnetic flux, as well the implications
of our work for the initiation of fast CMEs. *Work supported by NASA
and the Center for Integrated Space Weather Modeling (an NSF Science
and Technology Center).
Title: The Thermal Structure of AR 8038 During the May 1997 Event
Authors: Lionello, R.; Linker, J. A.; Mikić, Z.; Mok, Y.
Bibcode: 2005AGUSMSH51C..07L
Altcode:
During May 1997 active region AR 8038 was the site from which all solar
flares originated. In particular, on May 12 a halo CME was associated
with the only major flare of the day. We present a study of the thermal
structure of AR 8038 obtained using our computational MHD model. Our
algorithm solves the resistive and viscous full MHD equations in
3D. The energy equation includes thermal conduction parallel to the
magnetic field, a radiation loss term, and parametric heating. The
surface magnetic flux is prescribed as boundary condition. The initial
potential field is sheared to obtain an energized configuration. Then we
advance the MHD/thermodynamic model and look for a steady state. From
the plasma properties it is possible to calculate emissions in the
extreme ultraviolet and X-ray bands and compare the images with the
observations. Work supported by NASA and the Center for Integrated
Space Weather Modeling (an NSF Science and Technology Center).
Title: Structural Analysis of the Pre-Eruptive Magnetic Field for
the May 12, 1997 CME Event*
Authors: Titov, V. S.; Mikic, Z.; Linker, J. A.; Lionello, R.;
Riley, P.
Bibcode: 2005AGUSMSH54B..06T
Altcode:
A simple analytical model of the coronal magnetic field prior to
the CME eruption on May 12, 1997 is developed in the current-free
approximation. The magnetic field is constructed by superimposing a
large-scale background field and a localized bipole field to model the
active region. The background field is determined from the normal
component of the observed photospheric magnetic field averaged
over the longitude of the Sun. The influence of the solar wind is
taken into account by imposing a source-surface boundary condition
that makes the field radial at a specified radius. The field of the
active region is modeled with the help of a subphotospheric dipole
whose strength, location, and orientation are optimized to fit the
magnetic field obtained from an MDI magnetogram. A corresponding
force-free magnetic field is developed then by shearing and twisting the
potential configuration. The structure of the potential and force-free
configurations is analyzed and related to the characteristics of
the observed eruption. *Research supported by NASA and the Center
for Integrated Space Weather Modeling (an NSF Science and Technology
Center).
Title: Calculating the Thermal Structure of Solar Active Regions in
Three Dimensions
Authors: Mok, Yung; Mikić, Zoran; Lionello, Roberto; Linker, Jon A.
Bibcode: 2005ApJ...621.1098M
Altcode:
We describe a technique to obtain the temperature and density
distribution in an active region for a specified plasma heating
model. The technique can be applied in general to determine the magnetic
field and thermal structure self-consistently. For simplicity, we
illustrate the application of this technique in the limit of small
plasma β, in which the plasma dynamics decouples from that of the
magnetic field, a good approximation in active regions, in which
the magnetic field is strong. We select a particular active region,
observed in 1996 August, to demonstrate the methodology. We apply the
technique to a force-free magnetic field with a plasma heating model in
which the volumetric coronal heating rate is directly proportional to
the strength of the local magnetic field, and we compute the expected
extreme-ultraviolet and soft X-ray emissions from the resulting
thermal structure. We compare our solutions with one-dimensional
loop models and analytic loop scaling laws. In the future, we plan
to compare these emission images with those obtained by the SOHO EUV
Imaging Telescope (EIT) and the Yohkoh Soft X-Ray Telescope (SXT)
and to explore the relationship between coronal emission and various
coronal heating models.
Title: Relationship Between Plasma Heating and Coronal Emissions
Authors: Mok, Y.; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2004AGUFMSH13A1159M
Altcode:
The physical mechanisms that deposit energy into the coronal plasma
and dissipate it as heat have been speculated for decades, and yet
remain inconclusive. Many theoretical models have been proposed and
need to be validated by observation. To distinguish these models,
we take advantage of the complex magnetic-field structure and the
spatial inhomogeneity of an active region, and compute the thermal
structure in 3-D using the heating mechanism from each of the proposed
theories. Distinctive thermal structures are expected from different
heating models. As a result, each model has a unique signature in its
emissions in EUV and soft X-ray that are observable by EIT/SOHO and
SXT/Yohkoh. In our study, we survey the available models and compare
their predicted emissions with observations. Work supported by Sun
Earth Connection Theory Program of NASA
Title: What Can We Deduce about Coronal Mass Ejections from STEREO
observations?
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.; Odstrcil,
D.
Bibcode: 2004AGUFMSH21D..03L
Altcode:
Coronal mass ejections (CMEs) have been a topic of considerable
scientific interest for many years. Despite years of study, many
questions remains unanswered. What is the 3D structure of CMEs? How do
we relate the white-light images of CMEs near the Sun to the in situ
properties of ICMEs? What initiates CMEs? The STEREO mission promises
to yield an impressive array of images and in situ measurements that
can be brought to bear on these questions. The challenges facing the
solar and heliospheric community is to utilize this data to unravel
the structure and dynamics of CMEs. In this talk we will use 3D MHD
simulations of CMEs to illustrate some of the challenges we face in
deducing the structure of CMEs from 3 different viewpoints. We will
discuss how models may help to reduce these uncertainties. Research
supported by NASA and NSF.
Title: Using global MHD simulations to relate the Three-Part Structure
of CMEs to in situ observations
Authors: Riley, P.; Linker, J. A.; Mikic, Z.; Odstrcil, D.; Webb,
D. F.; Zurbuchen, T. H.
Bibcode: 2004AGUFMSH24A..07R
Altcode:
The classic three-part structure of Coronal mass ejections (CMEs)
observed in white light observations consisting of a bright
front, cavity, and dense core has generally been interpreted as
swept-up material, magnetic flux rope, and prominence/filamentary
material. Usually, however, there is no clear relationship between
these observations and in situ observations of CMEs, or magnetic
clouds, in particular. In this study we use a coupled coronal and
heliospheric MHD model to relate these disparate observations. The
simulations, while idealized, reproduce many of the features found in
solar observations, and suggest that the spacecraft's trajectory through
the CME plays a dominant role in the type of signatures observed. To
track these features through the heliosphere, we use an ensemble of
tracer particles. We also exploit the mapping of these tracer particles
to explore how compositional profiles associated with the CME evolve
as they move away from the Sun.
Title: Modeling Flux Cancellation in Active Regions
Authors: Mikic, Z.; Linker, J. A.; Titov, V.; Riley, P.; Lionello, R.
Bibcode: 2004AGUFMSH21B0401M
Altcode:
We will present a study of the flux cancellation mechanism in a model of
an active region. We will explore the interaction of the local magnetic
field in the active region with the global magnetic fields due to the
weak surrounding magnetic flux. The goal will be to see if a localized
3D configuration erupts as a consequence of flux cancellation, in the
same way that 2D axisymmetric and large-scale 3D fields erupt. Research
supported by NASA and the Center for Integrated Space Weather Modeling
(an NSF Science and Technology Center).
Title: MHD Modeling of Differential Rotation
Authors: Lionello, R.; Linker, J. A.; Mikić, Z.; Riley, P.
Bibcode: 2004AGUFMSH13A1157L
Altcode:
In the Fisk model differential rotation on the solar photosphere is
thought to cause large excursions of magnetic field lines in the upper
corona and in the heliosphere. This is in contrast with the apparently
rigid rotation of some coronal holes. The model of Fisk et al. (1999)
consists of a dipole field with the magnetic axis distinct from the
rotation axis. The field from the northern and southern polar holes
superexpands into the solar wind. In order to reconcile the pattern of
velocity in the upper corona with the photospheric differential flow,
magnetic reconnection is invoked between open field lines and low-lying
loops. This mechanism releases the plasma trapped in the closed field
lines into the corona and originates the slow solar wind. We have used
our MHD model in spherical coordinates to study this mechanism. We have
imposed a magnetic flux distribution as in Fisk et al. (1999). After
relaxing the system to steady state, we have applied differential
rotation for the equivalent of 5 rotations. We will describe the changes
in the coronal magnetic field in response to the photospheric flows.
Title: Coupled model simulation of a Sun-to-Earth space weather event
Authors: Luhmann, Janet G.; Solomon, Stanley C.; Linker, Jon A.; Lyon,
John G.; Mikic, Zoran; Odstrcil, Dusan; Wang, Wenbin; Wiltberger,
Michael
Bibcode: 2004JASTP..66.1243L
Altcode: 2004JATP...66.1243L
This paper describes the 3D simulation of a space weather event using
the coupled model approach adopted by the Center for Integrated Space
Weather Modeling (CISM). The simulation employs corona, solar wind,
and magnetosphere MHD models, and an upper atmosphere/ionosphere
fluid dynamic model, with interfaces that exchange parameters
specifying each component of the connected solar terrestrial system. A
hypothetical coronal mass ejection is launched from the Sun by a
process emulating photospheric field changes such as are observed
with solar magnetographs. The associated ejected magnetic flux rope
propagates into a realistically structured solar wind, producing
a leading interplanetary shock, sheath, and magnetic cloud. These
reach 1 AU where the solar wind and interplanetary magnetic field
parameters are used to drive the magnetosphere-ionosphere-thermosphere
coupled model in the same manner as upstream in situ measurements. The
simulated magnetosphere responds with a magnetic storm, producing
enhanced convection and auroral energy inputs to the upper
atmosphere/ionosphere. These results demonstrate the potential for
future studies using a modular, systemic numerical modeling approach
to space weather research and forecasting.
Title: Initial coupling of coronal and heliospheric numerical
magnetohydrodynamic codes
Authors: Odstrcil, Dusan; Pizzo, Victor J.; Linker, Jon A.; Riley,
Pete; Lionello, Roberto; Mikic, Zoran
Bibcode: 2004JASTP..66.1311O
Altcode: 2004JATP...66.1311O
Coupling of coronal and heliospheric magnetohydrodynamic (MHD) codes
represents an efficient modular approach to numerical simulations
of physical phenomena occurring on different spatial and temporal
scales. Using specialized models makes it possible to: (1) treat
efficiently the very different physics relevant in coronal and
interplanetary regimes; (2) deal with different time-step constraints
for coronal and heliospheric simulations; and (3) optimize and update
individual system components with the latest understanding. We present
results on the coupling of coronal and heliospheric numerical MHD codes
developed within the Center for Integrated Space weather Modeling. We
discuss 2-D parametric studies conducted to verify the computational
coupling procedure and to assess the accuracy of merging physically
and numerically different models.
Title: Fitting flux ropes to a global MHD solution: a comparison
of techniques
Authors: Riley, Pete; Linker, J. A.; Lionello, R.; Mikić, Z.;
Odstrcil, D.; Hidalgo, M. A.; Cid, C.; Hu, Q.; Lepping, R. P.; Lynch,
B. J.; Rees, A.
Bibcode: 2004JASTP..66.1321R
Altcode: 2004JATP...66.1321R
Flux rope fitting (FRF) techniques are an invaluable tool for
extracting information about the properties of a sub-class of coronal
mass ejections (CMEs) in the solar wind. However, it has proven
difficult to assess their accuracy since the underlying global
structure of the CME cannot be independently determined from the
data. In contrast, large-scale MHD simulations of CME evolution
can provide both a global view as well as localized time series
at specific points in space. In this study we apply five different
fitting techniques to two hypothetical time series derived from MHD
simulation results. Independent teams performed the analysis of the
events in "blind tests", for which no information, other than the
time series, was provided. From the results, we infer the following:
(1) Accuracy decreases markedly with increasingly glancing encounters;
(2) Correct identification of the boundaries of the flux rope can be a
significant limiter; and (3) Results from techniques that infer global
morphology must be viewed with caution. In spite of these limitations,
FRF techniques remain a useful tool for describing in situ observations
of flux rope CMEs.
Title: The photospheric boundary of Sun-to-Earth coupled models
Authors: Abbett, W. P.; Mikić, Z.; Linker, J. A.; McTiernan, J. M.;
Magara, T.; Fisher, G. H.
Bibcode: 2004JASTP..66.1257A
Altcode: 2004JATP...66.1257A
The least understood component of the Sun-to-Earth coupled system
is the solar atmosphere—the visible layers of the Sun that
encompass the photosphere, chromosphere, transition region and
low corona. Coronal mass ejections (CMEs), principal drivers of
space weather, are magnetically driven phenomena that are thought to
originate in the low solar corona. Their initiation mechanism, however,
is still a topic of great debate. If we are to develop physics-based
models with true predictive capability, we must progress beyond
simulations of highly idealized magnetic configurations, and develop
the techniques necessary to incorporate observations of the vector
magnetic field at the solar photosphere into numerical models of the
solar corona. As a first step toward this goal, we drive the SAIC
coronal model with the complex magnetic fields and flows that result
from a sub-photospheric MHD simulation of an emerging active region. In
particular, we successfully emerge a twisted Ω-loop into a pre-existing
coronal arcade. To date, it is not possible to directly measure
the magnetic field in the solar corona. Instead, we must rely on
non-potential extrapolations to generate the twisted, pre-eruptive
coronal topologies necessary to initiate data-driven MHD simulations
of CMEs. We therefore investigate whether a non-constant-α force-free
extrapolation can successfully reproduce the magnetic features of a
self-consistent MHD simulation of flux emergence through a stratified
model atmosphere. We generate force-free equilibria from simulated
photospheric and chromospheric vector magnetograms, and compare these
results to the MHD calculation. We then apply these techniques to an IVM
(Mees Solar Observatory) vector magnetogram of NOAA active-region 8210,
a source of a number of eruptive events on the Sun.
Title: Magnetohydrodynamic Modeling of Interplanetary CMEs
Authors: Riley, P.; Linker, J. A.; Mikic, Z.; Odstrcil, D.
Bibcode: 2004ITPS...32.1415R
Altcode:
Heliospheric models of coronal mass ejection (CME) propagation and
evolution provide an important insight into the dynamics of CMEs
and are a valuable tool for interpretating interplanetary in situ
observations. Moreover, they represent a virtual laboratory for
exploring conditions and regions of space that are not conveniently
or currently accessible by spacecraft. We summarize our recent
advances in modeling the properties and evolution of CMEs in the
solar wind. We describe our current state of research with three
examples: 1) interpreting the global context of in situ observations;
2) identifying new phenomena in the simulations; and 3) computing
geoeffective phenomena. We conclude by discussing what topics will
likely be important for models to address in the future.
Title: Effects of Heating Mechanism and Field Line Topology on the
Thermal Structure of an Active Region
Authors: Mok, Y.; Lionello, R.; Mikic, Z.; Linker, J. A.
Bibcode: 2004AAS...204.5608M
Altcode: 2004BAAS...36..763M
The thermal structure above an active region is strongly influenced
by the complex magnetic field through two mechanisms, plasma heating
and thermal conduction. The field line topology determines the path
of the heat flow, and the varying cross-section of the flux tube
affects its flow rate. Magnetic energy is believed to be the ultimate
source responsible for heating the plasma, although the conversion
mechanism is not yet understood. We chose Active Region 7986 (August,
1996) to study in details the influence of the magnetic field on
the thermal structure. Based on the magnetogram, we have constructed
several possible magnetic topologies for this region. Together with
these field structures, we use a number of plasma heating models to
construct the possible thermal structures in 3-D. The resulting EUV
and Soft X-ray emissions are computed from these models to show their
possible appearance and measure their resemblance to observations.
Title: Modeling Coronal Mass Ejections
Authors: Linker, J. A.; Mikic, Z.; Riley, P.; Lionello, R.
Bibcode: 2004AAS...204.6706L
Altcode: 2004BAAS...36..784L
Coronal mass ejections (CMEs) are immense eruptions of plasma and
magnetic field that are propelled outward from the sun. CMEs are an
important aspect of Space Weather at Earth, as they are believed to be
the primary cause of nonrecurrent geomagnetic storms. In this talk,
we compare eruptions generated by the "flux cancellation" (Linker et
al., Phys. Plasmas 10, 1971, 2003) and "breakout" (Antiochos et al.,
ApJ 512, 985, 1999) models and discuss how these mechanisms might be
distinguished by coronal and interplanetary observations. Work
supported by NASA and the Center for Integrated Space Weather Modeling
(an NSF Science and Technology Center).
Title: Modelling CMEs in the Heliosphere
Authors: Riley, P.; Linker, J. A.; Mikic, Z.; Odstrcil, D.
Bibcode: 2004cosp...35.1011R
Altcode: 2004cosp.meet.1011R
Heliospheric models of Coronal Mass Ejection (CME) propagation and
evolution provide an important insight into the dynamics of CMEs
and are a valuable tool for interpretating interplanetary in situ
observations. Moreover, they represent a virtual laboratory for
exploring conditions and regions of space that are not conveniently
or currently accessible by spacecraft. In this talk we summarize our
recent advances in modeling the properties and evolution of CMEs in the
solar wind. In particular, we focus on: interpretation of the global
context of in situ observations; identification of new phenomena in the
simulations; computation of geo-effective phenomena; and simulations
of specific case events. We conclude by discussing what topics will
likely be important for models to address in the future.
Title: Comparison of Observed Coronal EUV and X-Ray Emission with
that from Heating Models
Authors: Lionello, R.; Mok, Y.; Linker, J. A.; Mikić, Z.
Bibcode: 2003AGUFMSH42D..06L
Altcode:
The problem of finding the physical mechanism that heats the solar
corona is still unsolved. Many theoretical and observational models
have been proposed in the literature. In order to understand which
model better reproduces the observations, we present a quantitative
comparison between the emission calculated from different heating
models and the observed images of an active region. This investigation
uses our 3D MHD model in Cartesian coordinates, which calculates the
magnetic configuration of Active Region 7986 (August, 1996) starting
from a photospheric magnetogram, and another algorithm that solves
the 3D fluid equations along magnetic field lines, and that includes
thermal conduction, radiation losses, and the heating mechanism under
investigation. Once the plasma properties are found, the emission in
different wavelengths can be calculated using the Solarsoft package and
can be compared with the photon counts recorded by the EIT instrument
aboard the SOHO spacecraft, and the SXT instrument aboard the Yohkoh
satellite.
Title: Eruptive Behavior Originating in Active Regions
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.; Amari, T.
Bibcode: 2003AGUFMSH41A..04L
Altcode:
Coronal mass ejections (CMEs) are spectacular manifestations of
solar acitivy. These immense eruptions of plasma and magnetic field
are propelled outward from the sun with velocities as high as 2000
km/s. The fastest CMEs typically originate from active regions on the
Sun. MHD models of the eruption of large scale coronal fields have
demonstrated significant energy release in idealized 2D (Antiochos
et al., ApJ 512, 985, 1999) and 3D (Linker et al., Phys. Plasmas 10,
1971, 2003) geometry. Eruptive behavior has also been shown for a 3D
localized arcade (Amari et al., ApJ 529, L49, 2000). In active regions
on the Sun, both the localized field due to the active region and the
overlying fields in the large scale corona are important. We describe
MHD computations of the eruptive behavior of a localized active region
field (modeled as a localized bipole) within a large-scale dipolar
configuration. We discuss the differences between this more realistic
configuration and the idealized configurations that have been considered
previously. Work supported by NASA and the Center for Integrated Space
Weather Modeling (an NSF Science and Technology Center).
Title: Constraints on the structure and evolution of the coronal
magnetic field from in situ observations
Authors: Riley, P.; Linker, J. A.; Mikic, Z.
Bibcode: 2003AGUFMSH41D..01R
Altcode:
In this talk we briefly review current theories of the large-scale
heliospheric magnetic field. We address how measurements of the
coronal magnetic field can be connected to in situ observations
through numerical models, and likewise, how in situ observations can
be connected back to both solar observations and model results. We
focus on deviations from the ideal Parker spiral (e.g., radial field
lines, under-winding, over-winding, magnetic flux variations, transient
phenomena, etc) from near-Earth spacecraft as well as Ulysses, and ask
to what extent these observations can place constraints on theories
of the structure and evolution of the coronal magnetic field.
Title: Modeling the Large-Scale Corona Surrounding an Active Region
Authors: Mikic, Z.; Linker, J. A.; Lionello, R.; Riley, P.
Bibcode: 2003AGUFMSH42B0510M
Altcode:
Recent advances in our 3D MHD computational model have extended
our capability to study the detailed structure of an active region,
particularly how it is embedded in the magnetic field of surrounding
large-scale coronal streamers. Our spherical MHD code now gives us the
capability of concentrating the mesh points in an active region, where
the magnetic field is strong, while at the same time modeling the weaker
field of the large-scale corona of the whole Sun. It is thus possible
to study the interaction of the active-region magnetic field with that
of the Sun's overlying large-scale dipolar magnetic field. We will
show the active-region/streamer structure for particular simulations
of the solar minimum corona. These kind of simulations will be the
starting point for the study of fast CMEs that originate in active
regions. Research supported by NASA and the Center for Integrated
Space Weather Modeling (an NSF Science and Technology Center).
Title: Coronal Mass Ejection: Initiation, Magnetic Helicity, and
Flux Ropes. II. Turbulent Diffusion-driven Evolution
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.; Mikic, Z.; Linker, J.
Bibcode: 2003ApJ...595.1231A
Altcode:
We consider a three-dimensional bipolar magnetic field B, occupying
a half-space, which is driven into evolution by the slow turbulent
diffusion of its normal component on the boundary. The latter is imposed
by fixing the tangential component of the electric field and leads to
flux cancellation. We first present general analytical considerations on
this problem and then construct a class of explicit solutions in which
B keeps evolving quasi-statically through a sequence of force-free
configurations without exhibiting any catastrophic behavior. Thus,
we report the results of a series of numerical simulations in which B
evolves from different force-free states, the electric field on the
boundary being imposed to have a vanishing electrostatic part (the
latter condition is not enforced in the analytical model, and thus it is
possible a priori for the results of the two types of calculations to
be different). In all the cases, we find that the evolution conserves
the magnetic helicity and exhibits two qualitatively different
phases. The first one, during which a twisted flux rope is created,
is slow and almost quasi-static, while the second one is associated
with a disruption, which is confined for a small initial helicity and
global for a large initial helicity. Our calculations may be relevant
for modeling the coronal mass ejections that have been observed to
occur in the late dispersion phase of an active region. In particular,
they may allow us to understand the role played by a twisted flux rope
in these events.
Title: Three-Dimensional Magnetohydrodynamics of the Solar Corona
and of the Solar Wind with Improved Energy Transport
Authors: Lionello, Roberto; Linker, Jon A.; Mikić, Zoran
Bibcode: 2003AIPC..679..222L
Altcode:
We have developed a three-dimensional magnetohydrodynamic (MHD) model
of the solar corona and of the solar wind. We specify a magnetic flux
distribution on the solar surface and integrate the time dependent MHD
equations to steady state. The model originally employed a polytropic
energy equation. In order to improve the physics in our algorithm,
we have incorporated thermal conduction along the magnetic field,
radiation losses, and heating into the energy equation. The 2D version
of the model is able to reproduce the contrast in density between the
open and closed magnetic structures in the corona and the fast and slow
streams of the solar wind. We now present preliminary results of 3D MHD
simulations with improved thermodynamics. The results can be tested
against observations by spacecraft and Earth based observatories, in
situ solar wind and magnetic field measurements, heliospheric current
sheet crossings.
Title: Understanding the Solar Sources of In Situ Observations
Authors: Riley, Pete; Mikic, Zoran; Linker, Jon; Zurbuchen, Thomas H.
Bibcode: 2003AIPC..679...79R
Altcode:
The solar wind can, to a good approximation be described as a
two-component flow with fast, tenuous, quiescent flow emanating from
coronal holes, and slow, dense and variable flow associated with
the boundary between open and closed magnetic fields. In spite of
its simplicity, this picture naturally produces a range of complex
heliospheric phenomena, including the presence, location, and
orientation of corotating interaction regions and their associated
shocks. In this study, we apply a two-step mapping technique,
incorporating a magnetohydrodynamic model of the solar corona, to
bring in situ observations from Ulysses, WIND, and ACE back to the
solar surface in an effort to determine some intrinsic properties of
the quasi-steady solar wind. In particular, we find that a ``layer''
of ~35,000 km exists between the Coronal Hole Boundary (CHB) and the
fast solar wind, where the wind is slow and variable. We also derive a
velocity gradient within large polar coronal holes (that were present
during Ulysses' rapid latitude scan) as a function of distance from
the CHB. We find that v = 713 km/s + 3.2 d, where d is the angular
distance from the CHB boundary in degrees.
Title: 3-D simulations of ICMEs by coupled coronal and heliospheric
models
Authors: Odstrcil, D.; Riley, P.; Linker, J. A.; Lionello, R.; Mikic,
Z.; Pizzo, V. J.
Bibcode: 2003ESASP.535..541O
Altcode: 2003iscs.symp..541O
We overview the main features and approximations of the heliospheric
modeling system based on a 3-D ideal magnetohydrodynamic (MHD) model
that can be driven by various analytic, empirical, and numerical
models of the solar corona. Results are presented for two different
self-consistent simulations of Sun-to-Earth transient events: (1)
propagation of an interplanetary magnetic flux rope driven by the 3-D
MHD coronal model; and (2) the May 12-15, 1997 interplantary event
driven by an empirical model of the ambient solar wind and fitted
parameters for the halo coronal mass ejection (CME).
Title: Models of Coronal Mass Ejections: A Review with A Look to
The Future
Authors: Linker, Jon A.; Mikić, Zoran; Riley, Pete; Lionello, Roberto;
Odstrcil, Dusan
Bibcode: 2003AIPC..679..703L
Altcode:
Coronal mass ejections (CMEs) are a major transient input of mass and
energy into the solar wind. We review some of the past and present
concepts that influence the development of models of coronal mass
ejections, both for CME initiation and CME evolution and propagation
in the solar wind. We use the flux cancellation model to illustrate
present research on CMEs. Primarily for convenience, modeling of CME
propagation has usually been treated separately from the initiation
problem. We suggest that future computational modeling of interplanetary
CMEs is likely to emphasize the need to study coronal initiation and
solar wind propagation together.
Title: Using an MHD simulation to interpret the global context of
a coronal mass ejection observed by two spacecraft
Authors: Riley, Pete; Linker, J. A.; Mikić, Z.; Odstrcil, D.;
Zurbuchen, T. H.; Lario, D.; Lepping, R. P.
Bibcode: 2003JGRA..108.1272R
Altcode:
In late February 1999 the ACE spacecraft observed a coronal mass
ejection (CME) at 1 AU, in the ecliptic plane. Thirteen days later,
Ulysses observed a CME at 5 AU and 22°S. We present a detailed
analysis of the plasma, magnetic field, and composition signatures
of these two events. On the basis of this comparison alone, it is not
clear that the two spacecraft observed the same solar event. However,
using a generic MHD simulation of a fast CME initiated at the Sun by
magnetic flux cancellation and propagated out into the solar wind,
together with additional evidence, we argue that indeed the same CME
was observed by both spacecraft. Although force-free models appear to
fit the observed events well, our simulation results suggest that the
ejecta underwent significant distortion during its passage through the
solar wind, indicating that care should be taken when interpreting the
results of force-free models. Comparison of composition measurements
at the two spacecraft suggests that significant spatial inhomogeneities
can exist within a single CME.
Title: Dynamical evolution of the inner heliosphere approaching
solar activity maximum: interpreting Ulysses observations using a
global MHD model
Authors: Riley, P.; Mikic, Z.; Linker, J. A.
Bibcode: 2003AnGeo..21.1347R
Altcode:
In this study we describe a series of MHD simulations covering the
time period from 12 January 1999 to 19 September 2001 (Carrington
Rotation 1945 to 1980). This interval coincided with: (1) the Sun's
approach toward solar maximum; and (2) Ulysses' second descent to
the southern polar regions, rapid latitude scan, and arrival into
the northern polar regions. We focus on the evolution of several key
parameters during this time, including the photospheric magnetic field,
the computed coronal hole boundaries, the computed velocity profile
near the Sun, and the plasma and magnetic field parameters at the
location of Ulysses. The model results provide a global context for
interpreting the often complex in situ measurements. We also present a
heuristic explanation of stream dynamics to describe the morphology of
interaction regions at solar maximum and contrast it with the picture
that resulted from Ulysses' first orbit, which occurred during more
quiescent solar conditions. The simulation results described here are
available at: http://sun.saic.com.
Title: Parametric Dependence of Coronal Heating Mechanisms and
Active-Region Emissions
Authors: Mok, Y.; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2003SPD....34.0403M
Altcode: 2003BAAS...35..810M
The thermal structure of an active region depends on the mechanism
that heats the coronal plasma. A number of coronal heating mechanisms
have been proposed over the years. They have different parametric
dependences on the magnetic field, plasma density, and possibly other
variables. Different mechanisms result in different thermal structures,
and therefore, different EUV and soft X-ray emissions from an active
region. Hence, the comparison between the computed emissions based
on these models and the observed emissions will help to discover
the parametric dependences of the actual heating mechanism and put
some restrictions on the theoretical models. We have developed a 3D
thermo-magnetohydrodynamic code to compute the thermal structure of an
active region. The emissions resulted from various heating models will
be compared with the images obtained from SOHO and Yohkoh. This
work is supported by the Sun-Earth Connection Theory Program of NASA.
Title: Coronal Mass Ejection: Initiation, Magnetic Helicity, and
Flux Ropes. I. Boundary Motion-driven Evolution
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.; Mikic, Z.; Linker, J.
Bibcode: 2003ApJ...585.1073A
Altcode:
In this paper we study a class of three-dimensional magnetohydrodynamic
model problems that may be useful to understand the role of twisted flux
ropes in coronal mass ejections. We construct in a half-space a series
of force-free bipolar configurations with different helicity contents
and bring them into an evolution by imposing to their footpoints on
the boundary slow motions converging toward the inversion line. For
all the cases that have been computed, this process leads, after a
phase of quasi-static evolution, to the formation of a twisted flux
rope by a reconnection process and to the global disruption of the
configuration. In contrast with the results of some previous studies,
however, the rope is never in equilibrium. It thus appears that
the presence of a rope in the preeruptive phase is not a necessary
condition for the disruption but may be the product of the disruption
itself. Moreover, the helicity keeps an almost constant value during the
evolution, and the problem of the origin of the helicity content of an
eruptive configuration appears to be that of the initial force-free
state. In addition to these numerical simulations, we report some
new relations for the time variations of the energy and the magnetic
helicity and develop a simple analytical model in which the magnetic
field evolution exhibits essential features quite similar to those
observed during the quasi-static phase in the numerics.
Title: Acceleration region of the slow solar wind in corona
Authors: Abbo, L.; Antonucci, E.; Mikić, Z.; Riley, P.; Dodero,
M. A.; Giordano, S.
Bibcode: 2003MmSAI..74..733A
Altcode:
We present the results of a study concerning the physical parameters of
the plasma of the extended corona in the low-latitude and equatorial
regions, in order to investigate the sources of the slow solar wind
during the minimum of solar activity. The equatorial streamer belt has
been observed with the Ultraviolet Coronagraph Spectrometer (UVCS)
onboard SOHO from August 19 to September 1, 1996. The spectroscopic
diagnostic technique applied in this study, based on the OVI 1032,
1037 Ålines, allows us to determine both the solar wind velocity and
the electron density of the extended corona. The main result of the
analysis is the identification of the acceleration region of the slow
wind, whose outflow velocity is measured in the range from 1.7 up to
3.5 solar radii.
Title: 3-D MHD simulations of CMEs by coupled coronal and heliospheric
models
Authors: Odstrcil, D.; Linker, J. A.; Lionello, R.; Mikic, Z.; Riley,
P.; Pizzo, V. J.; Luhmann, J. G.
Bibcode: 2002ESASP.506...95O
Altcode: 2002ESPM...10...95O; 2002svco.conf...95O
Merging of coronal and heliospheric magnetohydrodynamic (MHD) models is
demonstrated for a 3-D idealized case involving a magnetic flux rope,
shock, streamer belt, and current sheet. The disruption of a sheared
helmet streamer launches a coronal mass ejection (CME, simulated by
the coronal model), which evolves during its propagation through
interplanetary space (simulated by the heliospheric model). These
models employ different physical approximations and numerical grids to
simulate physical phenomena over their respective spatial and temporal
domains. The coupled simulations enables self-consistent tracking of
transient disturbances from their origin in the solar atmosphere to
their geoeffective consequences at the Earth.
Title: CME Evolution in the Corona and Solar Wind
Authors: Linker, J. A.; Mikic, Z.; Riley, P.; Lionello, R.; Odstrcil,
D.
Bibcode: 2002AGUFMSH61A0433L
Altcode:
Coronal mass ejections (CMEs), immense eruptions of plasma and magnetic
fields with velocities as high as 2000 km/s, are a major transient input
of mass and energy into the solar wind. We are using time-dependent 2D
and 3D MHD computations to study the evolution of CMEs. A simulated CME
is initiated by flux cancellation at the photosphere. The calculation
follows the eruption and the subsequent propagation of a magnetic flux
rope to 1 AU. We discuss the evolutionary properties of the CME, both
near the Sun and beyond the Alfven and sonic points. We also examine
the properties likely to be inferred about the flux rope from simulated
time series data obtained when the flux rope passes over hypothetical
spacecraft at different positions. Work supported by NASA and NSF.
Title: Predicting the Structure of the Solar Corona During the
December 4, 2002 Total Solar Eclipse
Authors: Mikic, Z.; Linker, J. A.; Lionello, R.; Riley, P.
Bibcode: 2002AGUFMSH52A0468M
Altcode:
We describe the application of a three-dimensional magnetohydrodynamic
(MHD) model to the prediction of the structure of the corona during the
total solar eclipse that is expected to occur on 4 December 2002. The
calculation uses the observed photospheric radial magnetic field as
a boundary condition. This model makes it possible to determine the
large-scale structure of the magnetic field in the corona, as well
as the distribution of the solar wind velocity, plasma density, and
temperature. We will use magnetic fields observed on the solar disk
prior to eclipse day to predict what the corona will look like during
the eclipse. The estimated coronal density and temperature will be
used to predict the plane-of-sky polarization brightness and emission
of UV radiation prior to the eclipse. The prediction will be posted
on our web site (http://haven.saic.com) prior to the eclipse.
Title: Magnetic Field Topology in Prominences
Authors: Lionello, Roberto; Mikić, Zoran; Linker, Jon A.; Amari, Tahar
Bibcode: 2002ApJ...581..718L
Altcode:
We present a study of the magnetic field lines of a prominence using
MHD and thermodynamic/hydrodynamic (TH) models. Previous modeling of
prominences has tended to emphasize either magnetic field modeling or
TH modeling in isolation. In this paper, we combine these approaches
to model a long-lived filament observed in 1996 August-September. In
our new approach, we (1) use magnetograms to prescribe the boundary
conditions for the magnetic flux in three-dimensional MHD simulations,
(2) show that observed magnetic flux changes can produce a fluxrope
and that the dipped (concave upward) portion of the field lines form
in the approximate location of the observed prominence, and (3) show
that TH computations, using the computed geometry of magnetic field
lines that are in three-dimensional MHD equilibrium, have condensations
forming in the dipped portions of the field lines.
Title: Merging of coronal and heliospheric numerical two-dimensional
MHD models
Authors: Odstrcil, D.; Linker, J. A.; Lionello, R.; Mikic, Z.; Riley,
P.; Pizzo, V. J.; Luhmann, J. G.
Bibcode: 2002JGRA..107.1493O
Altcode:
Space weather research requires investigation of a complex chain
of coupled dynamic phenomena occurring simultaneously on various
spatial and temporal scales between the Sun and Earth. Specialized
physically based numerical models have been developed to address
particular aspects of the entire system. However, an integrated
modeling approach is necessary to provide a complete picture suitable
for interpretation of various remote and in situ observations and for
development of forecasting capabilities. In this paper we demonstrate
merging of coronal and heliospheric MHD models for a two-dimensional
hypothetical case involving a magnetic cloud, shock, streamer belt,
and current sheet. The disruption of a sheared helmet streamer launches
a coronal mass ejection (CME) (simulated by the coronal model), which
evolves during its propagation through interplanetary space (simulated
by the heliospheric model). These models employ different physical
approximations and numerical grids to simulate physical phenomena
over their respective spatial and temporal domains. The merging of
the models enables accurate tracking of a CME from its origin in the
solar atmosphere to its arrival at Earth.
Title: Evidence of Posteruption Reconnection Associated with Coronal
Mass Ejections in the Solar Wind
Authors: Riley, Pete; Linker, J. A.; Mikić, Z.; Odstrcil, D.; Pizzo,
V. J.; Webb, D. F.
Bibcode: 2002ApJ...578..972R
Altcode:
Using a coupled 2.5-dimensional, time-dependent MHD model of the
solar corona and inner heliosphere, we have simulated the eruption and
evolution of a coronal mass ejection containing a flux rope all the way
from the Sun to 1 AU. Although idealized, we find that the simulation
reproduces many generic features of magnetic clouds. In this paper we
report on a new, intriguing aspect of these comparisons. Specifically,
the results suggest that jetted outflow, driven by posteruptive
reconnection underneath the flux rope, occurs and may remain intact
out to 1 AU and beyond. We present an example of a magnetic cloud with
precisely these signatures and show that the velocity perturbations are
consistent with reconnection outflow. We suggest that other velocity
and/or density enhancements observed trailing magnetic clouds may
be signatures of such reconnection and, in some cases, may not be
associated with prominence material, as has previously been suggested.
Title: The solar wind at solar maximum: comparisons of EISCAT IPS
and in situ observations
Authors: Breen, A. R.; Riley, P.; Lazarus, A. J.; Canals, A.; Fallows,
R. A.; Linker, J.; Mikic, Z.
Bibcode: 2002AnGeo..20.1291B
Altcode:
The solar maximum solar wind is highly structured in latitude,
longitude and in time. Coronal measurements show a very high degree
of variability, with large variations that are less apparent within
in situ spacecraft measurements. Interplanetary scintillation (IPS)
observations from EISCAT, covering distances from 20 to 100 solar
radii (RS), are an ideal source of information on the inner
solar wind and can be used, therefore, to cast light on its evolution
with distance from the Sun. Earlier comparisons of in situ and IPS
measurements under solar minimum conditions showed good large-scale
agreement, particularly in the fast wind. In this study we attempt
a quantitative comparison of measurements made over solar maximum
by EISCAT (20 100 RS) and the Wind and Ulysses spacecraft
(at 215 RS and 300 1000 RS, respectively). The
intervals studied were August September 1999, May 2000, September 2000
and May 2001, the last-named being the period of the second Ulysses
fast latitude scan. Both ballistic and when possible MHD/ballistic
hybrid models were used to relate the data sets, and we compare the
results obtained from these two mapping methods. The results of this
study suggest that solar wind velocities measured in situ were less
variable than those estimated from IPS measurements closer to the
Sun, with the greatest divergence between IPS velocities and in situ
measurements occurring in regions where steep longitudinal velocity
gradients were seen in situ. We suggest that the interaction between
streams of solar wind with different velocities leads to "smoothing"
of solar wind velocities between 30 60
Title: The Structure and Dynamics of the Solar Corona and Inner
Heliosphere
Authors: Mikic, Zoran
Bibcode: 2002STIN...0267741M
Altcode:
This report covers technical progress during the fourth quarter of
the second year of NASA Sun-Earth Connections Theory Program (SECTP)
contract "The Structure and Dynamics of the Solar Corona and Inner
Heliosphere," NAS5-99188, between NASA and Science Applications
International Corporation (SAIC), and covers the period May 16,
2001 to August 15, 2001. Under this contract SAIC and the University
of California, Irvine (UCI) have conducted research into theoretical
modeling of active regions, the solar corona, and the inner heliosphere,
using the MHD (magnetohydrodynamic) model.
Title: Modeling the heliospheric current sheet: Solar cycle variations
Authors: Riley, Pete; Linker, J. A.; Mikić, Z.
Bibcode: 2002JGRA..107.1136R
Altcode:
In this report we employ an empirically driven, three-dimensional
MHD model to explore the evolution of the heliospheric current sheet
(HCS) during the course of the solar cycle. We compare our results
with a simpler ``constant-speed'' approach for mapping the HCS
outward into the solar wind to demonstrate that dynamic effects can
substantially deform the HCS in the inner heliosphere (<~5 AU). We
find that these deformations are most pronounced at solar minimum and
become less significant at solar maximum, when interaction regions
are less effective. Although solar maximum is typically associated
with transient, rather than corotating, processes, we show that even
under such conditions, the HCS can maintain its structure over the
course of several solar rotations. While the HCS may almost always
be topologically equivalent to a ``ballerina skirt,'' we discuss
an interval approaching the maximum of solar cycle 23 (Carrington
rotations 1960 and 1961) when the shape would be better described as
``conch shell''-like. We use Ulysses magnetic field measurements to
support the model results.
Title: MHD Modeling of CMEs near the Sun
Authors: Riley, P.; Mikic, Z.; Linker, J. A.; Lionello, R.
Bibcode: 2002AAS...200.6508R
Altcode: 2002BAAS...34..752R
Coronal Mass Ejections (CMEs) are spectacular events involving the
expulsion of significant amounts of solar material and energy into
the heliosphere. In spite of their importance, the physical mechanisms
leading to their eruption are not well known. In this talk, we review
how the magnetohydrodynamic (MHD) fluid description is applied to model
the initiation and evolution of CMEs near the Sun. In particular, we
describe: how the ambient solar corona is first established; how energy
is stored in the corona prior to eruption; and how either magnetic
shear and/or flux cancellation can subsequently lead to the eruption
of magnetic flux ropes with different physical properties. Research
supported by NASA and NSF.
Title: 3-D Simulation of Thermal Structure of Solar Active Regions
Authors: Mok, Y.; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2002AAS...200.0212M
Altcode: 2002BAAS...34..641M
The density-temperature profile of the atmosphere above an active
region fundamentally affects its characteristic photon emissions,
including UV, EUV and soft X-ray. Measurements made by Yohkoh, SOHO,
and TRACE of the bright, magnetic-field aligned, plasma loops partially
reveal the possible combinations of local plasma density and temperature
of these magnetic structures. We have studied the underlying physics
that leads to the unique combination of density and temperature,
as well as their spatial dependence, of the plasma that gives rise
to the characteristic EM emissions. The physical mechanisms include
heat deposition, radiative cooling and the highly anisotropic, highly
nonlinear thermal conduction. The 3-D simulation is very demanding on
computing resources due the sharp temperature gradient in the lower
transition region that ultimately requires a physical resolution of
10 km in a domain of 100000 km. We have devised a method to simplify
the computation based on the physical assumption that the plasma beta
in an active region is sufficiently low that the flows are primarily
along the field lines. The 3-D results agree well with a heuristic
1-D loop model for the density- temperature profiles along individual
field lines. Using a vector magnetogram of an active region, we have
computed the magnetic field for both potential and force-free cases,
their respective thermal structures and soft X-ray emissions. * Work
supported by The Sun-Earth Connection Theory Program of NASA.
Title: A Three-dimensional MHD Model of Solar Corona and Solar Wind
with Improved Energy Transport
Authors: Lionello, R.; Linker, J. A.; Mikic, Z.
Bibcode: 2002AAS...200.0302L
Altcode: 2002BAAS...34R.641L
A three-dimensional magnetohydrodynamic (MHD) model of the solar
corona and of the solar wind has been developed. A magnetic flux
distribution is specified on the solar surface and the time dependent
MHD equations are integrated to steady state. The model originally
employed a polytropic energy equation. In order to improve the physics
in our algorithm, thermal conduction along the magnetic field, radiation
losses, and heating have been incorporated into the energy equation. The
2D version of the model is able to reproduce the contrast in density
between the open and closed magnetic structures in the corona and the
fast and slow streams of the solar wind. We now present preliminary
results of 3D MHD simulations with improved thermodynamics. The results
can be tested against observations by spacecraft and Earth based
observatories, in situ solar wind and magnetic field measurements,
heliospheric current sheet crossings.
Title: Modeling CMEs in the Corona and Solar Wind
Authors: Linker, J. A.; Mikic, Z.; Riley, P.; Lionello, R.
Bibcode: 2002AGUSMSH22D..02L
Altcode:
Coronal mass ejections (CMEs), immense eruptions of plasma and magnetic
fields with velocities as high as 2000 km/s, are a major transient
input of mass and energy into the solar wind. CMEs typically appear
as loop-like features that disrupt helmet streamers in the solar
corona. The interplanetary manifestation of CMEs is often, but not
always, as flux ropes or magnetic clouds. In this talk we will review
models of CMEs, from their initiation and evolution in the corona
to their propagation in the inner heliosphere. We will discuss
how "complexity" might arise in CMEs during their passage through
interplanetary space, either due to interactions with the ambient
solar wind or to collisions with other CMEs. Research supported by
NASA and NSF.
Title: The Large-Scale Variability of Solar Wind Streams
Authors: Riley, P.; Mikic, Z.; Linker, J. A.; Lionello, R.; Lazarus,
A. J.
Bibcode: 2001AGUFMSH32A0729R
Altcode:
Fast and slow solar wind are fundamentally different. In particular,
slow solar wind shows a significant and persistent variability
that is absent in the fast wind. It is generally believed that this
structure is driven by a combination of temporal and spatial variations,
however, its origin remains poorly understood. It has been suggested,
for example, that the super-radial expansion of magnetic field lines
near the Sun governs the speed (and hence variability) of the solar
wind. Alternatively (or in addition to), it has been proposed that
the combination of differential rotation of the photosphere, together
with non-radial expansion of the solar wind through approximately
rigidly-rotating coronal holes leads to the opening of formerly closed
magnetic structures thereby releasing plasma into the solar wind. In the
first mechanism, the variability of the slow solar wind is predominantly
due to spatial variations in the solar magnetic field, whereas in the
second mechanism, the process is sporadic and dynamically driven. In
this study we use a combination of 3-D, time-dependent MHD simulations,
remote solar measurements, and in situ observations to explore the
contribution of temporal and spatial variability to the overall
large-scale structure of the solar wind from the solar surface to 5 AU.
Title: How do emerging magnetic fields affect the solar coronal
field configuration?
Authors: LI, Y.; Luhmann, J. G.; Abbett, W.; Linker, J.; Lionello,
R.; Mikic, Z.
Bibcode: 2001AGUFMSH11C0719L
Altcode:
Experiments are carried out to study the coronal field response to an
emerging active region into a simple background global magnetic field
using potential field source surface models. The emerging active region
used is the radial component of the magnetic field of an emerging flux
rope from an ANMHD simulation. When the active region is emerging
into a dipole field, it introduces polar coronal hole extensions,
warps the source surface neutral lines, and changes the field line
connections. The active region internal field line connections are also
changed to be different from an isolated active region. The relative
strength of the background and active region affect the extent of the
changes that occur. The field distribution of the background global
field is important, and different background with the same emerging
active region may result in different coronal features. A few examples
of different background fields with the emerging active region will be
presented and compared. A global MHD simulation is also in preparation
using the same global magnetic field with the emerging active region
as the boundary condition.
Title: Magnetohydrodynamic modeling of prominence formation within
a helmet streamer
Authors: Linker, J. A.; Lionello, R.; Mikić, Z.; Amari, T.
Bibcode: 2001JGR...10625165L
Altcode:
We present a 2.5-D axisymmetric MHD model to self-consistently describe
the formation of a stable prominence that supports cool, dense material
in the lower corona. The upper chromosphere and transition region
are included in the calculation. Reducing the magnetic flux along
the neutral line of a sheared coronal arcade forms a magnetic field
configuration with a flux rope topology. The prominence forms when dense
chromospheric material is brought up and condenses in the corona. The
prominence sits at the base of a helmet streamer structure. The dense
material is supported against gravity in the dips of the magnetic field
lines in the flux rope. Further reduction in magnetic flux leads to
an eruption of the prominence, ejecting material into the solar wind.
Title: An empirically-driven global MHD model of the solar corona
and inner heliosphere
Authors: Riley, Pete; Linker, J. A.; Mikić, Z.
Bibcode: 2001JGR...10615889R
Altcode:
In this study we describe a three-dimensional MHD model of the
solar corona and heliosphere. We split the modeling region into two
distinct parts: the solar corona (1 solar radius, RS,
to 30RS) and the inner heliosphere (30RS to 5
AU). This combined model is driven solely by the observed line-of-sight
photospheric magnetic field and can thus provide a realistic global
picture of the corona and heliosphere for specific time periods of
interest. We use the model to illustrate heliospheric structure during
three different phases of the solar cycle: (1) Carrington rotation
(CR) 1913 (August 22, 1996, to September 18, 1996), which occurred
near solar minimum and overlapped the ``Whole Sun Month'' campaign;
(2) CR 1892 (January 27, 1995, to February 23, 1995), which occurred
during the declining phase of cycle 22 and coincided with the so-called
``Ulysses rapid latitude scan'' and (3) CR 1947 (March 7, 1999, to April
4, 1999), which occurred approximately 15 months before the predicted
maximum of solar cycle 23. We compare Ulysses and Wind observations with
the simulation for CR 1913 and compare Ulysses observations during its
traversal from pole to pole with CR 1892. We find that the simulations
reproduce the overall large-scale features of the observations. We
use the near-solar-maximum results to speculate on the structure
of the high-latitude solar wind that Ulysses will encounter during
its traversal of the southern and northern solar poles in 2000 and
2001, respectively. In particular, the results suggest that because
of the presence of equatorial coronal holes the ordered pattern of
corotating interaction region tilts and their associated shocks, which
was observed during Ulysses' initial southward excursion in 1992, will
likely disappear completely as Ulysses moves toward the south pole. We
anticipate that Ulysses will encounter fast streams but will not remain
within them for more than a fraction of a solar rotaton. Finally,
the simulations suggest that crossings of the heliospheric current
sheet will persist up to at least ~70° heliographic latitude.
Title: Nature of the boundary between open and closed magnetic
field line regions at the Sun revealed by composition data and
numerical models
Authors: Posner, Arik; Zurbuchen, Thomas H.; Schwadron, Nathan A.;
Fisk, Lennard A.; Gloeckler, George; Linker, Jon A.; Mikić, Zoran;
Riley, Pete
Bibcode: 2001JGR...10615869P
Altcode:
Recently, Fisk et al. [1999] have presented a theory that describes a
number of features of the large-scale coronal and heliospheric magnetic
field. This theory predicts large-scale transport of magnetic flux
across the boundaries of the polar coronal holes, which leads to
reconnection processes of open field lines with preliminary closed
magnetic structures. Reconnection processes reveal themselves in
solar wind composition data: Plasma released out of previously closed
magnetic field structures exhibits hotter charge state distributions
and has a tendency to be enriched by elements with low first ionization
potentials. The idea of reconnection at the boundaries of coronal
holes is not new. For example, Wang and Sheeley [1993] and Luhmann
et al. [1999] found evidence for that mechanism by comparison of
observations of the rotation and evolution of coronal holes with
potential field models of the solar corona. We use Ulysses Solar Wind
Ion Composition Spectrometer composition measurements and sophisticated
numerical models [Linker et al., 1999; Riley et al., 1999] to accurately
map these observations back to the solar surface. We then constrain the
thickness of the stream interface at the Sun and compare the location
of the source region with SOHO observations of the low corona. The
results are discussed in the context of the global structure of the
heliospheric magnetic field.
Title: Interaction of Two Magnetic Loops in the Solar Corona
Authors: Mok, Y.; Mikić, Z.; Linker, J.
Bibcode: 2001ApJ...555..440M
Altcode:
The solar corona is populated by a large number of semitoroidal
magnetic loops, some of which are sufficiently close to each
other within a neighborhood that the probability of loop-to-loop
interaction is not negligible. The interaction of two coronal loops
is studied using a three-dimensional numerical simulation. The
first loop is an established, current-carrying magnetic loop in
hydromagnetic equilibrium. The second loop dynamically emerges from
the photosphere in the same neighborhood. There are a large number
of possible configurations in a two-loop system regarding their
relative orientation, physical size, and directions of their toroidal
magnetic field and electric current. We present three representative,
but characteristically different, configurations whose interactions
result in releasing various amounts of energy stored in the magnetic
field. Using typical coronal parameters, some of them can take place in
a relatively short timescale and release sufficient energy to account
for a small flare.
Title: Are There Two Classes of Coronal Mass Ejections? A Theoretical
Perspective
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.
Bibcode: 2001AGUSM..SH31C04L
Altcode:
Coronal mass ejections (CMEs) are generally accepted as the cause of
nonrecurrent geomagnetic storms at Earth. Statistical compilations of
CME events have shown that CMEs launched in the corona can have a wide
variation in speeds (Hundhausen et al., JGR 99, 6543, 1994). The speed
of the CME at Earth and the presence or absence of an interplanetary
shockwave is an important component of the geoeffectiveness; therefore,
the mechanism(s) by which fast CMEs might be produced are considered
to be of particular importance. Recently, the examination of the
acceleration profiles of CMEs has led to the possible classification of
CMEs as either constant speed CMEs or constant acceleration (St. Cyr
et al., JGR 104, 12493, 1999; Sheeley et al., JGR 104, 24739, 1999). In
this talk, we will examine reasons why one might expect CMEs to show two
classes of acceleration profiles, and if they do, what the implications
are for models of CME initiation. Examples from MHD simulations of
CMEs for different initiation mechanisms will be used to illustrate
essential points. Research supported by NASA and Boston University's
Integrated Space Weather Modeling project (funded by NSF).
Title: Modeling of CMEs from Sun to Earth with Merged Coronal and
Heliospheric MHD Computations*
Authors: Odstrcil, D.; Pizzo, V. J.; Linker, J. A.; Lionello, R.;
Mikic, Z.; Riley, P.; Luhmann, J. G.
Bibcode: 2001AGUSM..SH62A02O
Altcode:
The initiation of coronal mass ejections (CMEs) and their subsequent
manifestation in interplanetary space is a topic of great interest in
coronal and heliospheric science that also has important implications
for understanding and predicting space weather at Earth. We demonstrate
the computation of a CME initiated in the corona and its subsequent
propagation to Earth orbit by the merging of coronal and heliospheric
numerical MHD models. These models employ different physical
approximations and numerical grids to simulate physical phenomena
as well as possible over their respective spatial and temporal
domains. The computation starts by developing a steady-state helmet
streamer configuration with a heliospheric current sheet separating
interplanetary magnetic fields of opposite polarity. The disruption
of the helmet streamer by magnetic flux cancellation launches a CME
(simulated by the coronal model) which evolves during its propagation
through interplanetary space (simulated by the heliospheric model). The
merging of the models enables accurate tracking of the CME from its
origin in the solar atmosphere to its geoeffective consequences at
the Earth. *This work is supported by Boston University's Integrated
Space Weather Modeling project, funded by NSF.
Title: Using global MHD simulations to interpret in situ observations
of CMEs
Authors: Riley, P.; Linker, J. A.; Lionello, R.; Mikic, Z.; Odstrcil,
D.; Pizzo, V. J.; Zurbuchen, T. H.; Lario, D. D.
Bibcode: 2001AGUSM..SH42A07R
Altcode:
In this study, we combine two MHD models to simulate the initiation,
propagation, and dynamic evolution of flux-rope-like CMEs through the
corona and out to 1 AU. The coronal model encompasses the region of
the solar corona from 1 Rs to 20 Rs, while the heliospheric
model encompasses 20 Rs to 1 AU. The CME is initiated in
the corona and propagates smoothly across the outer boundary of the
coronal solution and through the inner boundary of the heliospheric
solution. The model solutions show a rich complexity, which, given the
relative simplicity and idealization of the input conditions, bear a
strong resemblance to many observed events, and we use the simulation
results to infer the global structure of some of these observations. In
particular, we highlight an event that was observed by both Ulysses and
ACE in February/March, 1999. At this time, Ulysses was located at ~
5 AU and S 22 ° heliographic latitude; thus the two spacecraft were
separated significantly both in heliocentric distance and latitude. We
also use these simulations to separate dynamical effects from force-free
models of flux ropes in the solar wind.
Title: Predicting the Structure of the Solar Corona During the 21
June 2001 Total Solar Eclipse
Authors: Mikic, Z.; Linker, J. A.; Lionello, R.; Riley, P.
Bibcode: 2001AGUSM..SH41B24M
Altcode:
We describe the application of a three-dimensional magnetohydrodynamic
(MHD) model to the prediction of the structure of the corona during
the total solar eclipse that is expected to occur on 21 June 2001. The
calculation uses the observed photospheric radial magnetic field as
a boundary condition. This model makes it possible to determine the
large-scale structure of the magnetic field in the corona, as well
as the distribution of the solar wind velocity, plasma density, and
temperature. We will use magnetic fields observed on the solar disk
prior to eclipse day to predict what the corona will look like during
the eclipse. The estimated coronal density will be used to predict
the plane-of-sky polarization brightness prior to the eclipse.
Title: 3-D Modeling of Thermal Structure in Active Regions on the
Solar Surface
Authors: Mok, Y.; Lionello, R.; Mikic, Z.; Linker, J.
Bibcode: 2001AGUSM..SH41A20M
Altcode:
The thermal structure of a magnetically active region depends on a
complicated balance between plasma heating, radiative cooling and
the highly anisotropic thermal conduction guided by the magnetic
field. It is also affected by plasma convection if siphon flows exist
as a result of dynamic imbalance of pressure gradient, gravity and
magnetic force. The difficulty of the numerical simulation lies in
the wide ranges of density and temperature, separated by a narrow
transition region with enormous gradients. Early studies of 1-D models
(Mok et. al. 1991) provide a guidance on the thermal structure along
individual field lines. A slightly more advanced 2-D model (Mok and Van
Hoven 1993) produces a differential emission measure that is remarkably
consistent with observations on the quiet sun. Active regions, however,
require a 3-D model. We have implemented the necessary thermodynamics
into our 3-D MHD code for this study. By starting with a magnetogram of
an active region, we first establish an overlying magnetic structure. We
then compute the thermal structure in the atmosphere. One of the most
poorly understood physical processes in the energy balance is the
plasma heating. We have computed the thermal structure based on various
heating models and will compare their resulting emission measures. Mok,
Schnack, and Van Hoven, 1991, Solar Phys. 132, 95. Mok and Van Hoven,
1993, Solar Phys. 146, 5. Work supported by the Sun Earth Connection
Theory Program of NASA.
Title: Coronal Sunspot Magnetic Fields: Extrapolation vs. Direct
Observation
Authors: Leka, K. D.; White, S.; Mikic, Z.; Lee, J.
Bibcode: 2001AGUSM..SH31D03L
Altcode:
Direct observations of the coronal magnetic field strength is presently
only available using radio wavelengths. It is, however, common to
infer the morphology of coronal magnetic fields by extrapolating
in height from observations of photospheric magnetic fields. Both
methods have sources of uncertainty, and neither presently results in
the quantitative coronal vector magnetic field measurements required
to understand coronal dynamics. In this paper we combine radio (VLA)
observations of a region containing a large spot with simultaneous
highly accurate photospheric vector magnetic field measurements obtained
with the NSO/HAO Advanced Stokes Polarimeter and investigate the coronal
response to changes in the photospheric magnetic field as NOAA AR8535
(May 1999) crossed the solar disk.
Title: Including the Transition Region in Models of the Large-Scale
Solar Corona
Authors: Lionello, Roberto; Linker, Jon A.; Mikić, Zoran
Bibcode: 2001ApJ...546..542L
Altcode:
In traditional multidimensional models of the solar corona, the boundary
of the calculation closest to the solar surface is placed well into
the corona (at temperatures of 1-2×106 K). We describe
a large-scale MHD model that includes the transition region in the
calculation. In this model, we simulate the solar atmosphere from the
top of the chromosphere (at 20,000 K), through the transition region,
into the corona, and extending out into the inner heliosphere. Our
model includes parallel thermal conduction, coronal heating, and
radiation losses in the energy equation. For simplicity, we describe
a two-dimensional (axisymmetric) implementation of our model. This
model enables us to study the large-scale structure of the transition
region. In particular, we contrast the variation of the structure
of the transition region underneath a closed magnetic arcade with
that in an open-field region. We discuss how the inclusion of the
transition region and upper chromosphere into the model modifies
the time constants. We compare our results with a model in which
the calculation is started near the top of the transition region
(at 500,000 K) using a ``radiative energy balance'' condition, and
we find that the two models agree well in open-field regions and for
long loops. However, a model that includes the transition region is
required to properly model short loops in closed-field regions.
Title: Measurements of the solar wind over a wide range of
heliocentric distances - a comparison of results from the first
three Whole Sun Months
Authors: Breen, A. R.; Thompson, B. J.; Kojima, M.; Biesecker, D. A.;
Canals, A.; Fallows, R. A.; Linker, J. A.; Lazarus, A. J.; Lecinski,
A.; Mikic, Z.; Moran, P. J.; Williams, P. J. S.
Bibcode: 2000JASTP..62.1527B
Altcode: 2000JATP...62.1527B
Co-ordinated observations of the Sun and inner heliosphere using a
large number of space- and ground-based instruments were carried out
in August-September 1996, August 1998 and August-September 1999 as the
first, second and third Whole Sun Months. These observations provided
unprecedented cover of the Sun and inner heliosphere at solar minimum
(1996) and during the rising phase of the new solar cycle (1998,
1999). In this paper we review the observations made during the three
Whole Sun Months and consider the changes in the large-scale structure
of the heliosphere seen over the four years.
Title: Evidence for topological nonequilibrium in magnetic
configurations
Authors: Vainshtein, S. I.; Mikić, Z.; Rosner, R.; Linker, J. A.
Bibcode: 2000PhRvE..62.1245V
Altcode: 2000astro.ph..3274V
We use direct numerical simulations to study the evolution, or
relaxation, of magnetic configurations to an equilibrium state. We
use the full single-fluid equations of motion for a magnetized,
nonresistive, but viscous fluid; and a Lagrangian approach is used
to obtain exact solutions for the magnetic field. As a result, the
topology of the magnetic field remains unchanged, which makes it
possible to study the case of topological nonequilibrium. We find two
cases for which such nonequilibrium appears, indicating that these
configurations may develop singular current sheets.
Title: An MHD Model of the Solar Wind from the Upper Chromosphere
to 1 A.U.
Authors: Lionello, R.; Linker, J. A.; Mikić, Z.
Bibcode: 2000SPD....31.0234L
Altcode:
Magnetohydrodynamic (MHD) models of the corona and solar wind have
been successful in reproducing many aspects of coronal structure,
as evidenced by favorable comparisons with eclipse and coronagraph
observations. However, the models do not accurately reproduce the solar
wind velocity in interplanetary space. This deficiency of the models
can be traced to the simple (polytropic) energy equation assumed. To
model solar wind acceleration, a more sophisticated treatment of
thermodynamic processes in the corona and solar wind is required. We
have developed a computational model of the solar wind that includes
thermal conduction parallel to the magnetic field, radiation, coronal
heating, and Alfvén wave pressure. Thermal conduction in the model
is collisionally dominated in the inner corona and smoothly becomes
collisionless in the outer corona. We have performed a two-dimensional
simulation of the solar wind in a computational domain that encompasses
the upper chromosphere, the transition region, the corona, and the
interplanetary space up to 1 A.U. We have obtained a steady-state
solution that reproduces the observed pattern of speeds, densities,
and particle fluxes of the fast polar wind and the slow equatorial wind.
Title: An MHD model of the solar wind from the upper chromosphere
to 1 A.U.
Authors: Lionello, R.; Linker, J. A.; Mikić, Z.
Bibcode: 2000BAAS...32..817L
Altcode:
No abstract at ADS
Title: Modeling the Corona During ``Whole Sun Month 3''
Authors: Mikic, Z.; Linker, J. A.; Riley, P.; Lionello, R.
Bibcode: 2000SPD....31.0238M
Altcode: 2000BAAS...32..817M
The ``Whole Sun Month 3'' interval (Aug. -- Sept. 1999) affords an
excellent opportunity to compare theroretical models with multiple
coordinated observations of the Sun as it approaches solar maximum. This
is the counterpart and continuation of the effort begun in the original
Whole Sun Month interval (Aug. -- Sept. 1996) during solar minimum
conditions. We will apply MHD models to calculate both active-region
magnetic fields and global magnetic fields in the corona, and compare
these with solar observations.
Title: Modeling of the Magnetic Structure of the Heliosphere using
MHD Simulations
Authors: Riley, P.; Mikic, Z.; Linker, J. A.
Bibcode: 2000SPD....31.0904R
Altcode: 2000BAAS...32..841R
We present the results of three-dimensional, time-dependent, MHD
simulations of the solar corona and heliosphere emphasizing the global
structure of the interplanetary magnetic field. Such simulations provide
a contextual basis with which to interpret a variety of disparate
solar and interplanetary observations. We divide the modeling into two
regions: The solar corona (1 -- 30 solar radii) and the heliosphere
(30 solar radii -- 5 AU). The solar coronal component of the model
is driven by measurements of the line-of-sight magnetic field in the
photosphere, such as those produced by the National Solar Observatory at
Kitt Peak. Inputs to the heliospheric component of the model are derived
from the output of the solar coronal model. Thus we are able to derive
solar wind plasma and magnetic field parameters at 1 AU and beyond,
based soley on the observed photospheric magnetic field. We test the
model for a variety of input conditions over the previous solar cycle
by comparing spacecraft trajectories flown through the simulation with
in situ observations made by WIND, ACE, and Ulysses. In particular, we
compare the sector structure and magnetic field variations observed
by the spacecraft with the simulation results. We also compare
the morphology of the heliospheric current sheet predicted by these
simulations with a ballistic extrapolation of the neutral line. These
comparisons suggest that the simulations can reproduce the essential
large-scale plasma and magnetic field features of the interplanetary
medium under a variety of solar conditions.
Title: Photospheric Magnetic Flux Changes as a Trigger for Coronal
Mass Ejections
Authors: Linker, J. A.; Lionello, R.; Mikic, Z.; Amari, T.
Bibcode: 2000SPD....31.0281L
Altcode: 2000BAAS...32..825L
Eruptive solar phenomena, such as coronal mass ejections, are believed
to be initiated by the release of energy stored in the coronal magnetic
field. Nonpotential magnetic field structures with significant amounts
of free magnetic energy are known to exist in the corona. However,
the mechanism(s) by which this energy is released are not well
understood. Previously, we have shown that the emergence of new magnetic
flux can lead to disruption of a helmet streamer, with liberation of
a significant fraction of the stored magnetic energy. Specifically,
eruption can be triggered when magnetic flux of opposite polarity
emerges near the neutral line, canceling some of the ambient flux. Prior
to eruption, a stable flux rope forms, which is a candidate magnetic
structure for prominence support. In this paper, we will describe how
other changes to the photospheric magnetic flux can lead to essentialy
the same eruptive process, and we will discuss the relationship of these
changes to flux reduction. Using full thermodynamic MHD simulations of
global coronal structure, we will also show that chromospheric material
can indeed be trapped in flux rope structures and lifted against the
solar gravity. Research supported by NASA.
Title: Solar Wind and Magnetic field Observations During Whole Sun
Months 2 and 3
Authors: Lazarus, A. J.; Szabo, A.; Linker, J. A.; Mikic, Z.
Bibcode: 2000SPD....31.0240L
Altcode: 2000BAAS...32..818L
We present observations from the Wind spacecraft of the solar wind
and the interplanetary magnetic field made during the second and
third Whole Sun Month periods (12-25 Aug, 1998, CR1939 and 18 Aug-14
Sept, 1999, CR1953 ). We compare those measurements (extrapolated to
the solar surface) with synoptic charts of the photospheric magnetic
field made from the Wilcox Solar Observatory. In contrast to the clear
coronal hole structures seen during the first Whole Sun Month (CR1913),
we see multiple sources of higher speed wind during the approach to
solar maximum.
Title: Effects on Magnetic Structures by Disrupting Plasma Flows
and Surface Magnetic Fields
Authors: Mok, Y.; Linker, J. A.; Mikic, Z.
Bibcode: 2000SPD....31.0151M
Altcode: 2000BAAS...32..810M
We have studied the dynamic evolution of current carrying magnetic
structures responding to temporal changes of plasma flows and
magnetic-field strength on the solar surface. These non-potential
structures can be formed by a variety of mechanisms, such as plasma
flows that twist the initially potential field and flux emergence. A
commonly seen example is an arcade field with shear plasma motions at
the footpoints of the field lines that result in a narrow current layer
directly above the neutral line. We have found that magnetic tearing
and possibly eruption of the structure can take place if there is a
sudden change in surface plasma flows or magnetic field strength. This
simple mechanism can be used as a model for some of the violent events
in the corona.
Title: The Structure and Dynamics of the Solar Corona
Authors: Mikic, Zoran
Bibcode: 2000STIN...0034000M
Altcode:
This report covers technical progress during the third year of the
NASA Space Physics Theory contract "The Structure and Dynamics of the
Solar Corona," between NASA and Science Applications International
Corporation, and covers the period June 16, 1998 to August 15,
1999. This is also the final report for this contract. Under this
contract SAIC, the University of California, Irvine (UCI), and the Jet
Propulsion Laboratory (JPL), have conducted research into theoretical
modeling of active regions, the solar corona, and the inner heliosphere,
using the MHD model. During the three-year duration of this contract
we have published 49 articles in the scientific literature. These
publications are listed in Section 3 of this report. In the Appendix we
have attached reprints of selected articles. We summarize our progress
during the third year of the contract. Full descriptions of our work
can be found in the cited publications, a few of which are attached
to this report.
Title: A Twisted Flux Rope Model for Coronal Mass Ejections and
Two-Ribbon Flares
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.; Linker, J.
Bibcode: 2000ApJ...529L..49A
Altcode:
We present a new approach to the theory of large-scale solar
eruptive phenomena such as coronal mass ejections and two-ribbon
flares, in which twisted flux tubes play a crucial role. We show
that it is possible to create a highly nonlinear three-dimensional
force-free configuration consisting of a twisted magnetic flux rope
representing the magnetic structure of a prominence (surrounded by
an overlaying, almost potential, arcade) and exhibiting an S-shaped
structure, as observed in soft X-ray sigmoid structures. We also
show that this magnetic configuration cannot stay in equilibrium and
that a considerable amount of magnetic energy is released during its
disruption. Unlike most previous models, the amount of magnetic energy
stored in the configuration prior to its disruption is so large that
it may become comparable to the energy of the open field.
Title: Interplanetary Scintillation Measurements of the Solar Wind
Above Low-Latitude Coronal Holes
Authors: Breen, A. R.; Moran, P. J.; Williams, P. J. S.; Lecinski,
A.; Thompson, B. J.; Harra-Murnion, L. K.; Mikic, Z.; Linker, J. A.
Bibcode: 2000AdSpR..26..789B
Altcode:
Observations of interplanetary scintillation (IPS) made using the EISCAT
facility provide accurate measurements of solar wind velocity between 15
and 120 solar radii. In this paper we present observations of the solar
wind at low latitudes made between 1994 and 1997. Of 178 observations
of the solar wind at low heliographic latitudes, 112 showed evidence of
flow velocities significantly faster than the normal slow wind across a
portion of the ray-path. In all cases, these enhanced flow speeds were
clearly associated with coronal holes extending towards or across the
heliographic equator. Fast flow from very close to the heliographic
equator is clearly associated with equator-crossing coronal holes in
all cases, suggesting that discrete streams of fast wind observed at
low latitudes originate exclusively in coronal holes
Title: Predicting the Structure of the Solar Corona During the 11
August 1999 Total Solar Eclipse
Authors: Mikic, Z.; Linker, J. A.; Riley, P.; Linello, R.
Bibcode: 2000ASPC..205..162M
Altcode: 2000ltse.conf..162M
We describe the application of a three-dimensional magnetohydrodynamic
(MHD) model to the prediction of the structure of the corona during the
total solar eclipse of 11 August 1999. The calculation uses the observed
photospheric radial magnetic field as a boundary condition. This
model makes it possible to determine the large-scale structure of
the magnetic field in the corona, as well as the distribution of the
solar wind velocity, plasma density, and temperature. The density
was used to predict the plane-of-sky polarization brightness prior to
the eclipse. The prediction is compared with an eclipse image taken
in Turkey.
Title: Energetic Particle Signatures of a Corotating Interaction
Region from a High Latitude Coronal Hole: SOHO, Wind and Ulysses
Observations
Authors: Posner, A.; Bothmer, V.; Kunow, H.; Gosling, J. T.; Heber,
B.; Lazarus, A. J.; Linker, J. A.; Marsden, R. G.; Mikić, Z.;
Müller-Mellin, R.; Sanderson, T. R.; Szabo, A.; Thompson, B. J.
Bibcode: 2000AdSpR..26..865P
Altcode:
In mid 1996 the Comprehensive Suprathermal and Energetic Particle
Analyser (COSTEP) onboard the Solar and Heliospheric Observatory, at
1 AU in the ecliptic plane, detected recurrent periods of enhanced
MeV ions in association with a corotating interaction region
(CIR). Measurements of energetic ions from the Cosmic Ray and Solar
Particle Instrument/Low Energy Telescope (COSPIN/LET) onboard Ulysses
taken at 5 AU, at mid-northern heliographic latitudes, showed that
Ulysses encountered recurrent particle events during the same time
period. We used the solar wind speeds observed at both locations to
estimate the cor-responding solar source longitudes of the particle
events. These longitudes are related to warps of the Sun's large high
latitude northern coronal hole boundaries observed by SOHO's Extreme
Ultraviolet Imaging Telescope (EIT). The findings are supported
by threedimensional magnetohydrodynamic (MHD) calculations of the
footpoint positions of the magnetic field lines at both spacecraft. The
observations suggest that close to the Sun a superradial expansion of
the fast solar wind from the Sun's high latitude northern coronal hole
down to ecliptic latitudes is present
Title: Solar Cycle Variations and the Large-Scale Structure of the
Heliosphere: MHD Simulations
Authors: Riley, Pete; Linker, J. A.; Mikic, Z.; Lionello, R.
Bibcode: 2000IAUJD...7E..12R
Altcode:
We present time-dependent, three-dimensional MHD computations of the
large-scale structure of the solar corona and heliosphere (out to
5 AU). The primary input to the model is the observed line-of-sight
component of the photospheric magnetic field. We investigate several
time periods to illustrate variations in heliospheric structure
during different phases of the solar cycle. In particular, we
emphasize structure at the maximum of the solar activity cycle. We
compare the results of the simulations with Ulysses and WIND in situ
observations and interplanetary scintillation measurements. We find
that the simulations can reproduce the overall large-scale features
of the observations and we use the results to speculate on the nature
of the high-latitude solar wind that Ulysses will likely encounter
during its polar passes in 2000 and 2001. Our results suggest that,
due to the presence of equatorial coronal holes, the ordered pattern
of CIR tilts and their associated shocks, which was observed during
Ulysses initial southward excursion in 1992, will likely disappear as
Ulysses traverses toward higher latitudes. We anticipate that while
Ulysses will continue to encounter fast streams, they will not persist
for more than a fraction of a solar rotation. Finally, the simulations
suggest that crossings of the heliospheric current sheet will persist
up to approximately 70 degrees heliographic latitude.
Title: Magnetohydrodynamics of the Solar Corona and the Transition
Region
Authors: Lionello, R.; Linker, J. A.; Mikic, Z.
Bibcode: 1999ESASP.448.1181L
Altcode: 1999mfsp.conf.1181L; 1999ESPM....9.1181L
No abstract at ADS
Title: Active Region Emissions and Coronal Field Extrapolations
Authors: Lee, J.; White, S. M.; Kundu, M. R.; Mikic, Z.
Bibcode: 1999spro.proc...65L
Altcode:
With vector magnetographs set to fly on the Solar--B mission,
the extrapolation of photospheric magnetic fields into the corona
will be increasingly important. As the techniques of coronal field
extrapolations grow more sophisticated, we require a more powerful means
to test them and to make full use of the information they contain. Radio
data can play an important role in testing extrapolation methods. In
this paper, we discuss a new test of coronal field extrapolation using
the concept of field line connectivity. The motivating idea is that
temperature should be nearly uniform on a given magnetic field line
due to the rapid transport of physical quantities along field lines
in the corona. Optically--thick gyroresonance emission provides
the temperature on a surface of known magnetic field strength in
the corona. As a consequence, we may expect that radio intensities
observed at different frequencies at points connected by field
lines should show a good correlation. This suggests that a test
of a magnetic field extrapolation model is whether the field--line
connectivity it predicts shows such a correlation. A second application
of field--line connectivity is to try to understand the relationship
between physical quantities in the photosphere at the footpoints of
magnetic field lines and the heating process in the corona on the same
field lines. If a particular magnetic quantity, such as shear, plays
a role in coronal heating then one expects the coronal extension of
field lines passing through peaks in this quantity will show the highest
coronal temperatures. This idea can be used to test candidate coronal
heating mechanisms. We demonstrate these ideas using the combination of
high--resolution VLA observations of a complex active region together
with state--of--the--art nonlinear force--free field modeling.
Title: An iterative method for the reconstructionbreak of the solar
coronal magnetic field. I. Method for regular solutions
Authors: Amari, T.; Boulmezaoud, T. Z.; Mikic, Z.
Bibcode: 1999A&A...350.1051A
Altcode:
We present a method for reconstructing the coronal magnetic field,
assumed to be in a non-linear force-free state, from its values given
in the photosphere by vector magnetograph measurements. In this paper,
that is the first of a series, we propose a method that solves the
boundary value problem set in the functional space of regular solutions
(i.e., that do not contain current sheets). This is an iterative method
introduced by Grad and Rubin. It is associated with a well-posed
boundary-value problem. We present some results obtained with this
method on two exact solutions of the magnetostatic equations, used as
theoretical magnetograms. Unlike some other extrapolations methods,
that are associated with ill-posed boundary value problems, our method
allows extrapolation to arbitrarily large heights, with no blowing up
due to the presence in these methods of an intrinsic instability that
makes errors growing up exponentially.
Title: On the Role of Magnetic Flux Ropes for Helicity Injection
and Ejection in the Solar Corona
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.
Bibcode: 1999ASPC..184...70A
Altcode:
We present some work on the role of magnetic flux ropes for the
structure and dynamics of the corona in term of its magnetic helicity
content and redistribution for solar eruptive and non eruptive
events. We show that is possible to construct 3D MHD solutions that
represents the first type of models for prominence magnetic support
as highly twisted non linear force-free type configuration. We also
present some results for the disruption of twisted magnetic flux rope
in non eruptive and eruptive confined events.
Title: Mercury Sodium Atmosphere, Magnetosphere and Solar Wind
Authors: Killen, R. M.; Giles, B.; Potter, A. E.; Jackson, B. V.;
Linker, J.; Mikic, Z.
Bibcode: 1999DPS....31.1801K
Altcode:
Earth-based observations of Mercury's sodium atmosphere have revealed
large-scale spatial variations and rapid temporal variations
(e.g. Potter and Morgan, Adv. Space Res. 19, 1571, 1997; Potter,
A.E. et al., Planet. Space. Sci., In press, 1999). We show how
the observed brightness variations may be related to the topology
of Mercury's magnetosphere in response to solar wind variations at
Mercury's orbit. Using the Toffoletto-Hill magnetosphere model modified
for Mercury (JGR 98, 1339, 1993), we have calculated the structure of
the Hermean magnetosphere for November, 1997, corresponding to dates
of our sodium images obtained at the National Solar Observatory at
Kitt Peak, Arizona. The sodium images are reduced to column abundance
using a Chamberlain type atmosphere with optically thick radiative
transfer. Inputs to the magnetosphere model are solar wind density and
velocity, and interplanetary magnetic field (IMF). Solar wind density
and velocity at the orbit of Mercury are inferred from heliospheric
tomography using radio scintillation measurements (Jackson et al.,
Adv. Space Res. 20, 23, 1997; Kojima et al., JGR 103, 1981, 1998). The
IMF at Mercury's position is obtained from a model of the inner
heliosphere constrained with the solar magnetic field, in combination
with density and temperature profiles at the sun's surface (Linker et
al., JGR 104, 9809, 1999). We use the morphology of the magnetosphere
along with solar wind parameters to infer the rates of ion sputtering
of sodium. The total sodium column is the sum of ion-sputtered sodium
and the more slowly varying sources, photon-stimulated desorption and
meteoritic vaporization. We show that the variations in ion sputtering
and subsequent loss via ioniation may be responsible for rapid changes
in the observed brightness distribution in sodium emissions. Our goal is
to show how solar activity and Mercury's sodium exosphere are related.
Title: Mercury sodium atmosphere, magnetosphere and solar wind.
Authors: Killen, R. M.; Giles, B.; Potter, A. E.; Jackson, B. V.;
Linker, J.; Mikic, Z.
Bibcode: 1999BAAS...31.1102K
Altcode:
No abstract at ADS
Title: The Three-dimensional Coronal Magnetic Field during Whole
Sun Month
Authors: Gibson, S. E.; Biesecker, D.; Guhathakurta, M.; Hoeksema,
J. T.; Lazarus, A. J.; Linker, J.; Mikic, Z.; Pisanko, Y.; Riley, P.;
Steinberg, J.; Strachan, L.; Szabo, A.; Thompson, B. J.; Zhao, X. P.
Bibcode: 1999ApJ...520..871G
Altcode:
Combining models and observations, we study the three-dimensional
coronal magnetic field during a period of extensive coordinated
solar observations and analysis known as the Whole Sun Month (WSM)
campaign (1996 August 10-September 8). The two main goals of the WSM
campaign are addressed in this paper, namely, (1) to use the field
configuration to link coronal features observed by coronagraphs and
imaging telescopes to solar wind speed variations observed in situ and
(2) to study the role of the three-dimensional coronal magnetic field
in coronal force balance. Specifically, we consider how the magnetic
field connects the two fastest wind streams to the two regions that
have been the main foci of the WSM analysis: the equatorial extension
of the north coronal hole (known as the Elephant's Trunk) and the
axisymmetric streamer belt region on the opposite side of the Sun. We
then quantitatively compare the different model predictions of coronal
plasma and solar wind properties with observations and consider the
implications for coronal force balance and solar wind acceleration.
Title: The Solar Origin of Corotating Interaction Regions and Their
Formation in the Inner Heliosphere
Authors: Balogh, A.; Bothmer, V.; Crooker, N. U.; Forsyth, R. J.;
Gloeckler, G.; Hewish, A.; Hilchenbach, M.; Kallenbach, R.; Klecker,
B.; Linker, J. A.; Lucek, E.; Mann, G.; Marsch, E.; Posner, A.;
Richardson, I. G.; Schmidt, J. M.; Scholer, M.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.; Aellig, M. R.; Bochsler, P.; Hefti, S.;
Mikić, Z.
Bibcode: 1999SSRv...89..141B
Altcode:
Corotating Interaction Regions (CIRs) form as a consequence of the
compression of the solar wind at the interface between fast speed
streams and slow streams. Dynamic interaction of solar wind streams
is a general feature of the heliospheric medium; when the sources of
the solar wind streams are relatively stable, the interaction regions
form a pattern which corotates with the Sun. The regions of origin
of the high speed solar wind streams have been clearly identified
as the coronal holes with their open magnetic field structures. The
origin of the slow speed solar wind is less clear; slow streams may
well originate from a range of coronal configurations adjacent to,
or above magnetically closed structures. This article addresses
the coronal origin of the stable pattern of solar wind streams
which leads to the formation of CIRs. In particular, coronal models
based on photospheric measurements are reviewed; we also examine
the observations of kinematic and compositional solar wind features
at 1 AU, their appearance in the stream interfaces (SIs) of CIRs,
and their relationship to the structure of the solar surface and the
inner corona; finally we summarise the Helios observations in the
inner heliosphere of CIRs and their precursors to give a link between
the optical observations on their solar origin and the in-situ plasma
observations at 1 AU after their formation. The most important question
that remains to be answered concerning the solar origin of CIRs is
related to the origin and morphology of the slow solar wind.
Title: Three-dimensional Solutions of Magnetohydrodynamic Equationsfor
Prominence Magnetic Support: Twisted Magnetic Flux Rope
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.; Linker, J.
Bibcode: 1999ApJ...518L..57A
Altcode:
The search for a background magnetic configuration favorable for
prominence support has been given a great deal of attention for several
decades. The most recent theoretical studies seem to agree that a
promising candidate for the support of the dense and cooler prominence
material, which fulfills several of the theoretical and observational
requirements such as twist, shear along the neutral line, and dips,
is a magnetic flux rope. The most convincing models take an infinitely
long periodic configuration that consists of a linear constant-α
force-free magnetic field. These models, however, assume values of α
that are close to its maximum possible value. In this Letter, we report
our recent results, which show that it is indeed possible to produce a
configuration that consists of a twisted magnetic flux tube embedded in
an overlaying, almost potential, arcade such that high electric currents
(and therefore values of α) are confined to the inner twisted magnetic
flux rope. We present two MHD processes--corresponding to two different
types of boundary conditions--that produce such a configuration. Our
results show that the process associated variations of Bz
at the photospheric level by applying an electric field involving
diffusion is much more efficient for creating a structure with more
twist and dips.
Title: Interplanetary scintillation measurements of high-speed flow
in the low-latitude solar wind
Authors: Breen, A. R.; Biesecker, D.; Fallows, R. A.; Lecinski, A.;
Mikic, Z.; Moran, P. J.; Williams, P. J. S.
Bibcode: 1999AIPC..471..317B
Altcode: 1999sowi.conf..317B
Observations of interplanetary scintillation (IPS) made using the EISCAT
facility provide accurate measurements of solar wind velocity between
15 and 120 solar radii (R). In this paper we present observations
of the solar wind at low latitudes made between 1991 and 1998, with
particular emphasis on flow from trans-equatorial coronal holes.
Title: Eruption of Magnetic Structures in the Solar Corona
Authors: Mok, Y.; Mikic, Z.; Linker, J.
Bibcode: 1999AAS...194.1708M
Altcode: 1999BAAS...31..854M
Magnetic structures of various geometries, including loops and arcades,
are present in the solar corona. Observations indicate that the magnetic
field in some of these structures can be highly sheared, implying that
a substantial amount of non-potential field energy is stored in the
structure. If there is a physical mechanism that can induce a transition
to a lower-energy state, the magnetic energy can be released into
kinetic energy of plasma motions or thermal energy. We have studied the
interactions between highly sheared structures (loops and arcades) and
an emerging potential field structure by 3-D numerical simulations. It
was found that the emerging structure can induce an MHD instability
that leads to the release of magnetic energy into plasma kinetic energy
at a rate near the ideal MHD time scale. A specific example is the
eruption along the neutral line of a long, narrow, sheared arcade,
which can be used to model a prominence eruption or a coronal mass
ejection. This work was supported by NASA SPTP. Computational resource
was provided by the San Diego Supercomputer Center through NSF.
Title: Magnetohydrodynamic modeling of the global solar corona
Authors: Mikić, Zoran; Linker, Jon A.; Schnack, Dalton D.; Lionello,
Roberto; Tarditi, Alfonso
Bibcode: 1999PhPl....6.2217M
Altcode:
A three-dimensional magnetohydrodynamic model of the global solar corona
is described. The model uses observed photospheric magnetic fields
as a boundary condition. A version of the model with a polytropic
energy equation is used to interpret solar observations, including
eclipse images of the corona, Ulysses spacecraft measurements of the
interplanetary magnetic field, and coronal hole boundaries from Kitt
Peak He 10 830 Å maps and extreme ultraviolet images from the Solar
Heliospheric Observatory. Observed magnetic fields are used as a
boundary condition to model the evolution of the solar corona during
the period February 1997-March 1998. A model with an improved energy
equation and Alfvén waves that is better able to model the solar wind
is also presented.
Title: In-ecliptic CIR-associated energetic particle events and polar
coronal hole structures: SOHO/COSTEP observations for the Whole Sun
Month Campaign
Authors: Posner, Arik; Bothmer, Volker; Thompson, Barbara J.; Kunow,
Horst; Heber, Bernd; Müller-Mellin, Reinhold; Lazarus, Alan J.;
Szabo, Adam; Mikić, Zoran; Linker, Jon A.
Bibcode: 1999JGR...104.9881P
Altcode:
The Solar and Heliospheric Observatory (SOHO), in halo orbit around
the L1 Lagrangian point of the Sun-Earth system, combines a unique
set of instruments for studies of the Sun and the heliosphere. SOHO's
Comprehensive Suprathermal and Energetic Particle Analyser measures
in situ particles in the energy range 44 keV/particle to above 53
MeV/nucleon. For the time period of the Whole Sun Month Campaign in
mid 1996 we have identified recurrent energetic particle intensity
increases in association with corotating interaction regions (CIRs)
in the energy range <10 MeV. Solar wind measurements of the Wind
spacecraft were used to estimate the corresponding magnetic source
location in Carrington longitude for comparison of energetic particles
with synoptic maps of the lower corona, derived from images of SOHO's
Extreme-ultraviolet Imaging Telescope. The comparison reveals a close
relationship of latitudinal extensions of polar coronal holes, situated
in regions up to 40° away from the ecliptic, with CIR-associated
in-ecliptic particle events.
Title: Magnetohydrodynamic modeling of the solar corona during Whole
Sun Month
Authors: Linker, J. A.; Mikić, Z.; Biesecker, D. A.; Forsyth, R. J.;
Gibson, S. E.; Lazarus, A. J.; Lecinski, A.; Riley, P.; Szabo, A.;
Thompson, B. J.
Bibcode: 1999JGR...104.9809L
Altcode:
The Whole Sun Month campaign (August 10 to September 8, 1996) brought
together a wide range of space-based and ground-based observations
of the Sun and the interplanetary medium during solar minimum. The
wealth of data collected provides a unique opportunity for testing
coronal models. We develop a three-dimensional magnetohydrodynamic
(MHD) model of the solar corona (from 1 to 30 solar radii) applicable
to the WSM time period, using measurements of the photospheric
magnetic field as boundary conditions for the calculation. We compare
results from the computation with daily and synoptic white-light and
emission images obtained from ground-based observations and the SOHO
spacecraft and with solar wind measurements from the Ulysses and WIND
spacecraft. The results from the MHD computation show good overall
agreement with coronal and interplanetary structures, including the
position and shape of the streamer belt, coronal hole boundaries,
and the heliospheric current sheet. From the model, we can infer the
source locations of solar wind properties measured in interplanetary
space. We find that the slow solar wind typically maps back to near
the coronal hole boundary, while the fast solar wind maps to regions
deeper within the coronal holes. Quantitative disagreements between
the MHD model and observations for individual features observed during
Whole Sun Month give insights into possible improvements to the model.
Title: Interplanetary scintillation measurements of the solar
wind during Whole Sun Month: Comparisons with coronal and in situ
observations
Authors: Breen, A. R.; Mikic, Z.; Linker, J. A.; Lazarus, A. J.;
Thompson, B. J.; Biesecker, D. A.; Moran, P. J.; Varley, C. A.;
Williams, P. J. S.; Lecinski, A.
Bibcode: 1999JGR...104.9847B
Altcode:
Two-site observations of interplanetary scintillation using the
EISCAT facility can provide measurements of solar wind velocity at
any point in the heliosphere between 15 and 120 solar radii (R). In
this paper we discuss a series of observations made as part of the
Whole Sun Month campaign (August 10 to September 8 1996) and compare
the results with coronal data and in-situ measurements made during
the campaign. The results of the comparison revealed extremely good
agreement between solar wind speeds measured by IPS at 16-73 R and in
situ measurements at 213 R and beyond, both in the general morphology of
the solar wind and in the absolute velocities observed. These results
confirm that structures in the solar wind, originating in the corona,
preserve their form out to 910 R or more. Observations of fast solar
wind were always associated with coronal holes and slow wind with the
bright corona. Velocities intermediate between normal fast and slow
flow speeds are associated with interaction regions between fast and
slow flow and are also found above the boundaries of coronal holes.
Title: Initiation of Coronal Mass Ejections by Changes in Photospheric
Flux
Authors: Mikic, Z.; Linker, J. A.
Bibcode: 1999AAS...194.5906M
Altcode: 1999BAAS...31..918M
We investigate the effect of changes in photospheric magnetic fields on
the stability of helmet streamers and active region arcades. Changes
in the magnetic flux in the vicinity of the neutral line can lead to
disruption, with liberation of a significant fraction of the stored
magnetic energy. When the amount of emerged flux is below a threshold,
a stable equilibrium with a filament results. When the threshold is
exceeded, the configuration erupts and leaves the Sun with a substantial
amount of kinetic energy. This mechanism is a promising trigger for
launching CMEs.
Title: MHD Modeling of the Solar Wind Including the Transition Region
Authors: Lionello, R.; Linker, J. A.; Mikic, Z.
Bibcode: 1999AAS...194.1614L
Altcode: 1999BAAS...31Q.852L
The structure of the solar corona is dominated by the magnetic
field. The MHD polytropic model based on observed photospheric
fields captures many features of the large-scale corona. However the
simplified energy equation in those models fails to reproduce the
temperature structure of the corona and the observed contrast in speed
between the fast and slow solar wind. We present some recent results
obtained with our MHD model utilizing a more careful treatment of
the thermodynamics. Our model includes thermal conduction parallel
to the magnetic field, radiation, coronal heating, and Alfven wave
pressure. Thermal conduction is collisionally dominated in the inner
corona, where it is calculated with Spitzer's formula, and smoothly
becomescollisionless in the outer corona. At the base of the corona we
can either impose radiation balance or we can model the transition
region directly. In our simulations we specify a magnetic flux
distribution on the solar surface and integrate the time-dependent
MHD equations to steady state. The resulting solutions can be tested
against observed properties of the corona and of the solar wind.
Title: Magnetohydrodynamic models of solar coronal magnetic fields.
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.
Bibcode: 1999PPCF...41..779A
Altcode:
The authors present some results concerning the possibility of
determining the structure of solar active regions using measurements of
the vector magnetic field on the Sun's surface as boundary conditions
for the new numerical extrapolation codes. From these computations
the main features of these configurations, shear and twist (which are
particular forms of magnetic helicity), are then used as ingredients
to define model problems and solved for the magnetohydrodynamic
(MHD) analysis of solar eruptive phenomena, in which ejection (or
redistribution) of helicity occurs.
Title: A Test for Coronal Magnetic Field Extrapolations
Authors: Lee, Jeongwoo; White, Stephen M.; Kundu, Mukul R.; Mikić ,
Zoran; McClymont, A. N.
Bibcode: 1999ApJ...510..413L
Altcode:
As models for the physical properties of the corona above solar
active regions grow more sophisticated, we will require better
means for testing them. In this paper we discuss and apply such a
test to a magnetic field model for an active region. This test is
based on the expectation that the temperatures at different points
on a given magnetic field line should be well correlated because of
the rapid transport of heat along field lines in the corona. We use
radio observations of an active region to measure the temperatures
on field lines as they cross two isogauss surfaces (at 430 and
750 G) in the corona. The field lines and isogauss surfaces are
derived from a coronal magnetic field model obtained via a nonlinear
force-free field extrapolation of a photospheric vector magnetogram;
for comparison, we also investigate a potential-field extrapolation
of the same magnetogram. In a region in which strongly sheared fields
are present, the nonlinear force-free field model does indeed show
a good correlation between the temperatures in the two surfaces at
points on the same field line, while the potential-field model does
not. This diagnostic acts both as a test of the magnetic field model
as well as of the interpretation of the radio data, and we show how
this test can also aid in understanding the radio data.
Title: The Solar Origin of Corotating Interaction Regions and their
Formation in the Inner Heliosphere
Authors: Balogh, A.; Bothmer, V.; Crooker, N. U.; Forsyth, R. J.;
Gloeckler, G.; Hewish, A.; Hilchenbach, M.; Kallenbach, R.; Klecker,
B.; Linker, J. A.; Lucek, E.; Mann, G.; Marsch, E.; Posner, A.;
Richardson, I. G.; Schmidt, J. M.; Scholer, M.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.; Aellig, M. R.; Bochsler, P.; Hefti, S.;
Mikić, Z.
Bibcode: 1999cir..book..141B
Altcode:
No abstract at ADS
Title: MHD Modeling of the Solar Corona
Authors: Mikic, Zoran
Bibcode: 1998APS..DPPD2I204M
Altcode:
The structure of the solar corona is determined to a large extent
by the magnetic field. Unfortunately, it is not possible to measure
the magnetic field in the corona (in general). To maximize the
interpretation of solar observations it is therefore necessary
to use theoretical models. By using a three-dimensional MHD model
to describe the self-consistent interaction of magnetic, plasma,
and gravity forces, including the effect of the solar wind, it is
possible to determine the large-scale structure of the magnetic field
in the corona and inner heliosphere, as well as the distribution of
the solar wind velocity, plasma density, and temperature. The model
can provide quantitative outputs that can be tested directly against
observations, including coronagraph images, in situ solar wind and
magnetic field measurements, heliospheric current sheet crossings,
interplanetary scintillation measurements, and emissions in various
radiation lines. The principal input to the model is the observed
photospheric magnetic field. Comparisons between the model and eclipse
observations, coronagraph observations, SOHO data, and Ulysses and
WIND interplanetary plasma measurements will be presented. Research
sponsored by NASA and NSF. Computations performed at NERSC and SDSC.
Title: Sector boundary transformation by an open magnetic cloud
Authors: Crooker, N. U.; McAllister, A. H.; Fitzenreiter, R. J.;
Linker, J. A.; Larson, D. E.; Lepping, R. P.; Szabo, A.; Steinberg,
J. T.; Lazarus, A. J.; Mikic, Z.; Lin, R. P.
Bibcode: 1998JGR...10326859C
Altcode: 1998JGR...103R6859C
A magnetic cloud observed by the Wind spacecraft on February 8,
1995, was remarkable for its impact on the interplanetary sector
structure. The magnetic field data imply that the cloud occurred in
the middle of a sector and that the arrival of the following sector
boundary on February 10 coincided with the arrival time predicted from
the corresponding source surface map. The electron heat flux data,
however, give incontrovertible evidence that instead the cloud brought
the sector boundary, well ahead of the predicted arrival time. The
electron heat flux data show little counterstreaming within the cloud,
indicating predominantly open helical field lines. Under the assumption
that the cloud originally had the form of a closed flux rope loop with
legs rooted to the Sun, observational constraints dictate that the
sector boundary was displaced not because it was pushed aside by the
cloud but because reconnection in the leading leg opened field lines
there, creating a topological change spanning 45° of heliographic
longitude. The solar source of the cloud was deduced from an associated
eruptive arcade event extending northeastward from an active region
in Yohkoh soft X ray data on February 4. On February 8, the same
active region was the source of impulsive energetic electron events
observed at Wind during a brief counterstreaming interval, consistent
with magnetic connection in the leading leg at that time. The cloud's
helicity matches that predicted from the skew of the arcade fields in
the February 4 X ray event, but the predicted alignment of the arcade
and cloud axes was off by 35°. We use an MHD model with boundary
conditions derived from solar magnetograms to illustrate the tilted
arcade configuration in the corona that gave rise to the magnetic cloud
and the lesser tilt of the heliospheric current sheet stemming from it.
Title: Spatial structure of the solar wind and comparisons with
solar data and models
Authors: Neugebauer, M.; Forsyth, R. J.; Galvin, A. B.; Harvey,
K. L.; Hoeksema, J. T.; Lazarus, A. J.; Lepping, R. P.; Linker,
J. A.; Mikic, Z.; Steinberg, J. T.; von Steiger, R.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.
Bibcode: 1998JGR...10314587N
Altcode:
Data obtained by instruments on the Ulysses spacecraft during its rapid
sweep through >90° of solar latitude, crossing the solar equator
in early 1995, were combined with data obtained near Earth by the
Wind spacecraft to study the spatial structure of the solar wind and
to compare to different models of the interplanetary magnetic field
derived from solar observations. Several different source-surface
models matched the double sinusoidal structure of the heliospheric
current sheet (HCS) but with differences in latitude as great as
21°. The source-surface model that included an interplanetary
current sheet gave poorer agreement with observed current-sheet
crossings during this period than did the other source-surface models
or an MHD model. The differences between the calculated and observed
locations of the HCS were minimized when 22° of solar rotation was
added to the constant-velocity travel time from the source surface to
the spacecraft. The photospheric footpoints of the open field lines
calculated from the models generally agreed with observations in the
He 10,830 Å line of the locations of coronal holes with the exceptions
that (1) in some places, open field lines originated outside the coronal
hole boundaries and (2) the models show apparently closed-field regions
just inside some coronal hole boundaries. The patterns of mismatches
between coronal hole boundaries and the envelopes of open field lines
persisted over at least three solar rotations. The highest-speed wind
came from the polar coronal holes, with the wind originating deeper
within the hole being faster than the wind coming from near the
hole boundary. Intermediate and slow streams originated in smaller
coronal holes at low latitudes and from open field regions just
outside coronal hole boundaries. Although the HCS threaded regions
of low speed, low helium abundance, high ionization temperature,
and a high ratio of magnesium to oxygen densities (a surplus of an
element with low first-ionization potential), there was a great deal
of variation in these parameters from one place to another along
the HCS. The gradient of speed with latitude varied from 14 to 28
kms-1deg-1.
Title: Coronal Currents, Magnetic Fields, and Heating in a Solar
Active Region
Authors: Lee, Jeongwoo; McClymont, A. N.; Mikić, Zoran; White,
Stephen M.; Kundu, Mukul R.
Bibcode: 1998ApJ...501..853L
Altcode:
We compare microwave images of a solar active region with
state-of-the-art fully nonlinear force-free extrapolations of the
photospheric fields in order to study the link between coronal currents
and heating of the corona. This extrapolation fully takes into account
the nonuniform distribution of electric currents observed in the
photosphere and its role in the coronal magnetic structure. We carry
out the comparison for AR 6615, a complex region observed with the
VLA on 1991 May 7. Under the assumption that the microwave emission
is dominated by optically thick gyroresonance radiation, we may use
the radio images to infer the temperature of the corona at different
heights and locations. This is then compared with heating models based
on the observed current distribution. We are able to reproduce the radio
images remarkably well with a model in which temperature is structured
along magnetic field lines, depends on the current on the field line,
and increases with height in a manner similar to that inferred from
static heated loop models. This result implies a direct link between
electric currents and coronal heating.
Title: Microwave Mode Coupling Above Active Regions as a Coronal
Density Diagnostic
Authors: Lee, Jeongwoo; White, Stephen M.; Kundu, M. R.; Mikić,
Zoran; McClymont, A. N.
Bibcode: 1998SoPh..180..193L
Altcode:
It is well recognized that the phenomenon of depolarization (the
conversion of polarized radio emission into unpolarized emission) of
microwaves over solar active regions can be used to infer the coronal
electron density once the coronal magnetic field is known. In this
paper we explore this technique using an active region for which we
have excellent radio data showing depolarization at two frequencies,
and for which we have an excellent magnetic field model which has been
tested against observations. We show that this technique for obtaining
coronal densities is very sensitive to a number of factors. When Cohen's
(1960) theory where depolarization is due to magnetic field rotation
alone is used, the result is particularly sensitive to the location
of the surface on which the magnetic field is orthogonal to the line
of sight. Depending on whether we take into account the presence
of electric currents in the photosphere or not, their extrapolation
into the corona can result in very different heights being deduced
for the location of the depolarization strip, and this changes the
density which is then deduced from the depolarization condition. Such
extreme sensitivity to the magnetic field model requires that field
extrapolations be able to accurately predict the polarity of magnetic
fields up to coronal heights as high as ∼ 105 km in order
to exploit depolarization as a density diagnostic.
Title: The Structure and Dynamics of the Solar Corona
Authors: Mikic, Zoran
Bibcode: 1998saic.reptR....M
Altcode:
Under this contract SAIC, the University of California, Irvine (UCI),
and the Jet Propulsion Laboratory (JPL), have conducted research into
theoretical modeling of active regions, the solar corona, and the inner
heliosphere, using the MHD model. During the period covered by this
report we have published 17 articles in the scientific literature. These
publications are listed in Section 4 of this report. In the Appendix
we have attached reprints of selected articles.
Title: The Structure and Dynamics of the Solar Corona
Authors: Mikic, Zoran
Bibcode: 1998nasa.reptT....M
Altcode:
This report covers technical progress during the first year of the NASA
Space Physics Theory contract between NASA and Science Applications
International Corporation. Under this contract SAIC, the University
of California, Irvine (UCI), and the Jet Propulsion Laboratory
(JPL), have conducted research into theoretical modeling of active
regions, the solar corona, and the inner heliosphere, using the MHD
model. During the period covered by this report we have published 26
articles in the scientific literature. These publications are listed
in Section 4 of this report. In the Appendix we have attached reprints
of selected articles.
Title: Magnetohydrodynamics of solar coronal plasmas in cylindrical
geometry.
Authors: Lionello, R.; Mikic, Z.; Schnack, D. D.
Bibcode: 1998JCoPh.140..172L
Altcode:
The authors describe a three-dimensional algorithm for the advancement
of the resistive MHD equations in cylindrical geometry with line-tied
boundary conditions. This code has been developed to simulate the
behavior of solar coronal plasmas.
Title: Nonlinear Magnetohydrodynamic Evolution of Line-tied Coronal
Loops
Authors: Lionello, Roberto; Velli, Marco; Einaudi, Giorgio; Mikić,
Zoran
Bibcode: 1998ApJ...494..840L
Altcode:
Simulations of the nonlinear evolution of the m = 1 kink mode
in magnetic flux tubes with line-tying boundary conditions are
presented. The initial structure of the flux tube is intended to
model a solar coronal loop that either has evolved quasi-statically
through sequences of equilibria with increasing twist due to the
application of localized photospheric vortex flows or has emerged with
a net current through the photosphere. It is well known that when the
twist exceeds a critical value that depends on its radial profile and
on the loop length, the loop becomes kink unstable. The nonlinear
evolution of the instability is followed using a three-dimensional
MHD code in cylindrical geometry, in different types of magnetic field
configurations, with the common property that the current is confined
within the same radius, so that the magnetic field is potential
in the external regions. The differences reside in the net axial
current carried by the structure, ranging from a vanishing current
(corresponding to an outer axial potential field) to a high current
(corresponding to an outer almost azimuthal potential field). It
is shown that, during the nonlinear phase of the instability, loops
develop current sheets and, consequently, their evolution becomes
resistive with the occurrence of magnetic reconnection. The dependence
of the topology of the currents at saturation on the initial magnetic
structure, the details of the reconnection phenomenon, and the resistive
dissipation mechanism are examined. Finally, the impact of the results
on the understanding of coronal activity is discussed.
Title: The Emergence of Current-Carrying Magnetic Loops into the
Solar Corona
Authors: Mok, Y.; Van Hoven, G.; Mikić, Z.
Bibcode: 1997ApJ...490L.107M
Altcode:
The mechanism of the dynamic emergence of current-carrying
magnetic loops into the corona is examined by three-dimensional MHD
simulations. By simultaneously modeling the spacetime profiles of
the normal components of the emerging magnetic field and current
density on the photosphere, we demonstrate that this process can
qualitatively reproduce observations that show the emergence of a
helically twisted magnetic structure with a suitable field-current
combination. The flux-tube structure rises into the initially nearly
field-free corona and gradually relaxes into a nearly force-free,
steady state, magnetic loop.
Title: Reconstructing the Solar Coronal Magnetic Field as a Force-Free
Magnetic Field
Authors: Amari, T.; Aly, J. J.; Luciani, J. F.; Boulmezaoud, T. Z.;
Mikic, Z.
Bibcode: 1997SoPh..174..129A
Altcode:
We present some preliminary results on different mathematical
problems encountered in attempts to reconstruct the coronal magnetic
field, assumed to be in a force-free state, from its values in the
photosphere. We discuss the formulations associated with these problems,
and some new numerical methods that can be used to get their approximate
solutions. Both the linear constant-α and the nonlinear cases are
considered. We also discuss the possible use of dynamical 3D MHD
codes to construct approximate solutions of the equilibrium force-free
equations, which are needed for testing numerical extrapolation schemes.
Title: Reconstruction of the Three-Dimensional Coronal Magnetic Field
Authors: Jiao, Litao; McClymont, A. N.; MikiĆ, Z.
Bibcode: 1997SoPh..174..311J
Altcode:
Studies of solar flares indicate that the mechanism of flares is
magnetic in character and that the coronal magnetic field is a key to
understanding solar high-energy phenomena. In our ongoing research we
are conducting a systematic study of a large database of observations
which includes both coronal structure (from the Soft X-ray Telescope
on the Yohkoh spacecraft) and photospheric vector magnetic fields
(from the Haleakala Stokes Polarimeter at Mees Solar Observatory). We
compare the three-dimensional nonlinear force-free coronal magnetic
field, computed from photospheric boundary data, to images of coronal
structure. In this paper we outline our techniques and present results
for active region AR 7220/7222. We show that the computed force-free
coronal magnetic field agrees well with Yohkoh X-ray coronal loops,
and we discuss the properties of the coronal magnetic field and the
soft X-ray loops.
Title: Problems and Progress in Computing Three-Dimensional Coronal
Active Region Magnetic Fields from Boundary Data
Authors: McClymont, A. N.; Jiao, L.; MikiĆ, Z.
Bibcode: 1997SoPh..174..191M
Altcode:
An overview of the whole process of reconstructing the coronal magnetic
field from boundary data measured at the photosphere is presented. We
discuss the errors and uncertainties in the data and in the data
reduction process. The problems include noise in the magnetograph
measurements, uncertainties in the interpretation of polarization
signals, the 180° ambiguity in the transverse field, and the fact
that the photosphere is not force-free. Methods for computing the
three-dimensional structure of coronal active region magnetic fields,
under the force-free assumption, from these boundary data, are then
discussed. The methods fall into three classes: the `extrapolation'
technique, which seeks to integrate upwards from the photosphere
using only local values at the boundary; the `current-field iteration'
technique, which propagates currents measured at the boundary along
field lines, then iteratively recomputes the magnetic field due to
this current distribution; and the `evolutionary' technique, which
simulates the evolution of the coronal field, under quasi-physical
resistive magnetohydrodynamic equations, as currents injected at the
boundary are driven towards the observed values. The extrapolation
method is mathematically ill-posed, and must be heavily smoothed to
avoid exponential divergence. It may be useful for tracing low-lying
field lines, but appears incapable of reconstructing the magnetic field
higher in the corona. The original formulation of the current-field
iteration method had problems achieving convergence, but a recent
reformulation appears promising. Evolutionary methods have been applied
to several real datasets, with apparent success.
Title: 3-D Simulation of Current-Carrying Magnetic Loops Emerging
through the Photosphere
Authors: Mok, Y.; van Hoven, G.; Mikic, Z.
Bibcode: 1997SPD....28.0246M
Altcode: 1997BAAS...29..901M
We have investigated the emergence of current-carrying magnetic loops
into the solar corona. In previous works, we have denomstrated that a
magnetic loop in the corona can be formed by twisting the field lines of
an existing, in situ, bipolar potential field structure in the corona by
plasma convections. This action induces an electric current, primarily
in the toroidal direction, resulting in a magnetic loop with helical
field lines (Van Hoven et al. 1995). In the present work, we show
that a current-carrying magnetic flux rope emerges directly through
the solar surface from underneath the photosphere into an almost
field-free corona. This process involves a simultaneous injection
of both toroidal magnetic field and toroidal electric current into
the corona from the surface. By specifying the space-time profiles
of the normal components of the emerging magnetic field and the
current density at the base (photosphere), which can be obtained from
observations, our simulations show that a current-carrying flux rope
emerges from the surface as if it were driven by buoyancy from under the
photosphere. The flux rope rises into the corona and gradually relaxes
to a near force-free state. Multiple-loop interactions will also be
discussed. * Work supported, in part, by NASA SPT, NSF ATM and AFPL;
computational resources provided by NSF and UCI at SDSC, and by DOE
at NERSC. Van Hoven, G., Mok, Y. and Mikic, Z., Ap.J. 1995, 440, L105.
Title: Currents, Magnetic Fields and Heating in a Solar Active Region
Authors: Lee, J.; McClymont, A. N.; Mikic, Z.; White, S. M.; Kundu,
M. R.
Bibcode: 1997SPD....28.1602L
Altcode: 1997BAAS...29R.920L
We have compared high-quality microwave images of the radio emission
from the corona above an active region with state-of-the-art nonlinear
force-free extrapolations of the photospheric magnetic field. The radio
images, which are dominated by the opacity provided by the coronal
magnetic fields, show excess magnetic field in locations consistent
with the expected location of coronal currents. We test the hypothesis
that the degree of heating on a given coronal magnetic flux tube is
related to the current flowing through it by comparing model radio
brightness distributions at different frequencies with the actual
observations. In the model we assume that temperature is distributed
along the field lines according to quasi-static loop models, and that
there is effectively no diffusion across the field lines. This coronal
heating model is able to reproduce the radio brightness distributions
remarkably well.
Title: Mapping the Magnetic Structure of the Corona During the
ULYSSES Fast-Latitude Scan
Authors: Linker, J. A.; Mikic, Z.
Bibcode: 1997SPD....28.0601L
Altcode: 1997BAAS...29..909L
The coronal magnetic field defines the large-scale structure of
the solar corona, the position of the heliospheric current sheet,
and the regions of fast and slow solar wind. To understand the
structure of the solar corona and inner heliophere, we must relate
observations of the large-scale magnetic field at the photosphere to
coronal and interplanetary observations. Global magnetohydrodynamic
(MHD) models of the solar corona show great promise in accurately
mapping the photospheric magnetic field outwards from the Sun. Data
from February--April, 1995 (Carrington rotations 1892-1894) provide an
opportunity to test this capability, as the Ulysses spacecraft traversed
a wide range of heliographic latitudes, and, at the same time, the Mauna
Loa Coronameter made white-light observations of the inner corona. We
compare results from magnetohydrodynamic (MHD) models of the solar
corona during this time period with Mauna Loa data. By ``flying''
the Ulysses spacecraft trajectory through the model data, we produce
simulated traces of the magnetic field for the Ulysses fast-latitude
scan and directly compare these results with Ulysses observations.
Title: Reconstruction of the Three-Dimensional Coronal Magnetic Field
Authors: Jiao, Litao; McClymont, Alexander N.; Mikic, Z.
Bibcode: 1997SPD....28.0149J
Altcode: 1997BAAS...29..888J
Our analysis of active region AR7220/7222 has revealed some remarkable
features of the solar coronal magnetic field: The region of bright
X-ray loops is bounded by separatrix surfaces and restricted to regions
of short field lines. Surrounding areas conspicuously lacking in soft
X-ray emission are connected by long field lines to distant opposite
polarity. The force-free field lines align well with observed coronal
loops and agree better than potential field lines do. The footpoints
of the brightest set of loops lie in a photospheric flux tube which
has a very distinct sheath of return current. Furthermore, the bright
loops appear to have both ends rooted in quite strong magnetic field
(~ 500 G). The separatrix surfaces do not seem to contribute to heating
of the loops; the lengths of the field lines and field strength at the
footpoints seem to be the most significant factors. These findings
stimulate us to investigate the physics of loop heating in other
active regions. We reconstruct the coronal magnetic field of AR6919
from a series of three magnetograms which span the 1991 November 15
flare event. We also compare coronal fields of AR6919 computed from
two kinds of boundary conditions: one with alpha specified everywhere,
the other with alpha specified only over one polarity. We expand our
study of loop heating by computing loop brightnesses based on heating
fluxes dependent on the footpoint magnetic field strength, apply the
coronal loop scaling law to determine temperature and density, and
map these onto the computed field lines. We compute the X-ray flux
and integrate along the line-of-sight to obtain two-dimensional images.
Title: Determination of Coronal Magnetic Fields from Vector
Magnetograms
Authors: Mikic, Zoran
Bibcode: 1997saic.rept.....M
Altcode:
During the course of the present contract we developed an 'evolutionary
technique' for the determination of force-free coronal magnetic fields
from vector magnetograph observations. The method can successfully
generate nonlinear force- free fields (with non-constant-a) that
match vector magnetograms. We demonstrated that it is possible to
determine coronal magnetic fields from photospheric measurements,
and we applied it to vector magnetograms of active regions. We
have also studied theoretical models of coronal fields that lead to
disruptions. Specifically, we have demonstrated that the determination
of force-free fields from exact boundary data is a well-posed
mathematical problem, by verifying that the computed coronal field
agrees with an analytic force-free field when boundary data for the
analytic field are used; demonstrated that it is possible to determine
active-region coronal magnetic fields from photospheric measurements,
by computing the coronal field above active region 5747 on 20 October
1989, AR6919 on 15 November 1991, and AR7260 on 18 August 1992, from
data taken with the Stokes Polarimeter at Mees Solar Observatory,
University of Hawaii; started to analyze active region 7201 on 19 June
1992 using measurements made with the Advanced Stokes Polarimeter
at NSO/Sac Peak; investigated the effects of imperfections in the
photospheric data on the computed coronal magnetic field; documented
the coronal field structure of AR5747 and compared it to the morphology
of footpoint emission in a flare, showing that the 'high- pressure'
H-alpha footpoints are connected by coronal field lines; shown that the
variation of magnetic field strength along current-carrying field lines
is significantly different from the variation in a potential field,
and that the resulting near-constant area of elementary flux tubes is
consistent with observations; begun to develop realistic models of
coronal fields which can be used to study flare trigger mechanisms;
demonstrated that magnetic nonequilibrium can disrupt sheared coronal
arcades, and that helmet streamers can disrupt, leading to coronal mass
ejections. Our model has significantly extended the realism with which
the coronal magnetic field can be inferred from actual observations. In
a subsequent contract awarded by NASA, we have continued to apply and
improve the evolutionary technique, to study the physical properties
of active regions, and to develop theoretical models of magnetic fields.
Title: A Spacecraft Going Behind the Sun Will Support SOHO
Authors: Ruzmaikin, A.; Anderson, J. D.; Asmar, S.; Bird, M.; Cassiani,
A.; Coles, W.; Feynman, J.; Harvey, J.; Harvey, K.; Hollweg, J.;
Linker, K.; Mikic, Z.; Pätzold, M.; Smith, E. J.
Bibcode: 1997ESASP.404..653R
Altcode: 1997cswn.conf..653R
No abstract at ADS
Title: STEREO: a solar terrestrial event observer mission concept
Authors: Socker, Dennis G.; Antiochos, S. K.; Brueckner, Guenter E.;
Cook, John W.; Dere, Kenneth P.; Howard, Russell A.; Karpen, J. T.;
Klimchuk, J. A.; Korendyke, Clarence M.; Michels, Donald J.; Moses,
J. Daniel; Prinz, Dianne K.; Sheely, N. R.; Wu, Shi T.; Buffington,
Andrew; Jackson, Bernard V.; Labonte, Barry; Lamy, Philippe L.;
Rosenbauer, H.; Schwenn, Rainer; Burlaga, L.; Davila, Joseph M.; Davis,
John M.; Goldstein, Barry; Harris, H.; Liewer, Paulett C.; Neugebauer,
Marcia; Hildner, E.; Pizzo, Victor J.; Moulton, Norman E.; Linker,
J. A.; Mikic, Z.
Bibcode: 1996SPIE.2804...50S
Altcode:
A STEREO mission concept requiring only a single new spacecraft has been
proposed. The mission would place the new spacecraft in a heliocentric
orbit and well off the Sun- Earth line, where it can simultaneously view
both the solar source of heliospheric disturbances and their propagation
through the heliosphere all the way to the earth. Joint observations,
utilizing the new spacecraft and existing solar spacecraft in earth
orbit or L1 orbit would provide a stereographic data set. The new
and unique aspect of this mission lies in the vantage point of the
new spacecraft, which is far enough from Sun-Earth line to allow an
entirely new way of studying the structure of the solar corona, the
heliosphere and solar-terrestrial interactions. The mission science
objectives have been selected to take maximum advantage of this new
vantage point. They fall into two classes: those possible with the
new spacecraft alone and those possible with joint measurements using
the new and existing spacecraft. The instrument complement on the new
spacecraft supporting the mission science objectives includes a soft
x-ray imager, a coronagraph and a sun-earth imager. Telemetry rate
appears to be the main performance determinant. The spacecraft could
be launched with the new Med-Lite system.
Title: The Viability of Ohmic Dissipation as a Coronal Heating Source
Authors: Hendrix, D. L.; van Hoven, G.; Mikic, Z.; Schnack, D. D.
Bibcode: 1996ApJ...470.1192H
Altcode:
We have performed three-dimensional numerical simulations of the coronal
heating model proposed by Parker (1972,1994) and have studied the steady
state power balance between Poynting flux (P) and ohmic dissipation
(Q). We demonstrate that this power balance exists and how P and Q
scale with the driving velocity, granular coherence time, and loop
length. We show that both P and Q compare well with the Markovian limit
of the order-of-magnitude estimate given by Parker (1983). Our results
further indicate a weak positive-exponent scaling with the Lundquist
(conductivity) number. These results imply that line-tied photospheric
convection can drive large enough current densities in the corona to
make Parker's mechanism feasible.
Title: The large-scale structure of the solar corona and inner
heliosphere
Authors: Mikić, Zoran; Linker, Jon A.
Bibcode: 1996AIPC..382..104M
Altcode:
The helmet streamers that are observed to dominate the structure of
the inner corona are formed by the interaction of the solar wind with
coronal magnetic fields. We have simulated this interaction in three
dimensions using the magnetohydrodynamic (MHD) equations. In order to
create a realistic model, we use the magnetic field that is observed
at the Sun's surface (deduced from daily Wilcox Solar Observatory
magnetic field synoptic maps) as input, in combination with specified
density and temperature profiles at the surface. A self-consistent
3D solar-wind solution is developed by integrating the MHD equations
in time to steady state. Such solutions can reproduce the observed
structures that are seen in coronagraph images and eclipse photographs
of the corona. We compare the results obtained from our model with
with an eclipse photograph of the corona on November 3, 1994. We also
compare the position of the heliospheric current sheet with Ulysses
observations during the period May-June 1993.
Title: An MHD Model of the Solar Corona and Solar Wind
Authors: Mikic, Z.; Linker, J. A.; Colborn, J. A.
Bibcode: 1996AAS...188.3307M
Altcode: 1996BAAS...28..868M
The structure of the heliosphere, especially the regions of fast and
slow solar wind, are strongly influenced by coronal magnetic structure
near the Sun. Favorable comparisons between three-dimensional MHD
models of the solar corona and eclipse observations have shown
that it is possible to model the structure of the large-scale solar
corona. However, these models use a simplified energy equation, in
which the plasma is assumed to obey an adiabatic energy equation
with a reduced polytropic index. As a consequence, even though
the predicted streamer structure in the corona agrees fairly well
with eclipse observations, the predicted solar wind speed is not
realistic. We have improved this model by adding important dynamic
and thermodynamic effects, including the presence of a transition
region, thermal conduction, radiation, coronal heating, and Alfven
wave acceleration. We will present results obtained with this improved
model on the structure of the solar corona and solar wind.
Title: Magnetic Loop Emergence in the Solar Atmosphere *
Authors: Mok, Y.; van Hoven, G.; Mikic, Z.
Bibcode: 1996AAS...188.3613M
Altcode: 1996BAAS...28Q.875M
We have studied various mechanisms that lead to the emergence of
magnetic loop structures into the solar atmosphere. In addition to
vortical photospheric motions that twist the existing coronal field into
magnetic loops (poloidal field injection, Van Hoven et al. 1995), we
examine the toroidal field/current injection mechanisms that directly
bring magnetic flux to the surface from the plasma underneath the
photosphere. By specifying a time-dependent emerging magnetic field
at the base (photosphere), our simulations show that a potential
field can be established in the corona. If electric current is also
injected into the atmosphere through the base, with an appropriate
relation to the magnetic field, a magnetic loop structure can rise
through the surface into the corona, and gradually relax into a near
force-free state. Multiple-loop interactions will also be discussed. *
Work supported, in part, by NASA SPT, NSF ATM and AFPL; computational
resources provided by NSF and UCI at SDSC, and by DOE at NERSC. Van
Hoven, G., Mok, Y. and Mikic, Z., Ap.J. 1995, 440, L105.
Title: Global Coronal Modeling and Space Weather Prediction
Authors: Linker, Jon A.; Mikic, Zoran; Schnack, Dalton D.
Bibcode: 1996ASPC...95..208L
Altcode: 1996sdit.conf..208L
No abstract at ADS
Title: Modeling of Active-Region Magnetic Fields
Authors: Mikic, Zoran; Linker, Jon A.; Schnack, Dalton D.
Bibcode: 1996ASPC...95..108M
Altcode: 1996sdit.conf..108M
No abstract at ADS
Title: Surface Driven Evolution and Activity of Atmospheric Magnetic
Structures
Authors: van Hoven, G.; Mok, Y.; Hendrix, D. L.; Mikic, Z.
Bibcode: 1996mpsa.conf...51V
Altcode: 1996IAUCo.153...51V
No abstract at ADS
Title: Large-scale structure of the solar corona and inner heliosphere
Authors: Mikic, Z.; Linker, J. A.
Bibcode: 1995sowi.conf...60M
Altcode:
The large-scale structure of the solar corona influences solar activity
particularly coronal mass ejections (CMEs). The helmet streamers
that are observed to dominate the structure of the inner corona are
formed by the interaction of the solar wind with coronal magnetic
fields. We have simulated this interaction in three dimensions using
the magnetohydrodynamic (MHD) equations. In order to create a realistic
model, we use the magnetic field that is observed at the Sun's surface
(deduced from daily Wilcox Solar Observatory magnetograms) as input,
in combination with specified density and temperature profiles at
the surface. A self-consistent 3D solar-wind solution is developed by
integrating the MHD equations in time to steady state. Such solutions
can reproduce the observed structures that are seen in coronagraph
images and eclipse photographs of the corona. This model allows us
to accurately determine the position of the heliospheric current
sheet. We will compare the results obtained from our model with
Ulysses observations during the period May-June 1993, and with an
eclipse photograph of the corona on November 3, 1994.
Title: Coronal mass ejections and the evolution of the large-scale
corona
Authors: Linker, J. A.; Mikic, Z.
Bibcode: 1995sowi.conf...60L
Altcode:
Coronal mass ejections (CMEs) are dynamic, large-scale events in
the solar corona that expel plasma and magnetic fields into the solar
wind. The structure of the large-scale corona influences CME occurrence;
for example, CMEs are most frequently observed to be disruptions
of coronal or helmet streamer configurations on the Sun. We have
investigated the evolution of the large-scale corona in the presence
of differential rotation, using time-dependent magnetohydrodynamic
(MHD) simulations in two and three dimensions. An equilibrium coronal
configuration is first computed by superimposing a Parker solar wind
flow on an initial potential field, the equations are integrated in
time until a steady-state is reached. When differential rotation
is introduced, we find that the configuration does not reach a
steady state; the closed field regions (helmet streamers) disrupt
recurrently, ejecting plasmoids into the solar wind. Our results
suggest that differential rotation may be one of the mechanisms by
which mass ejections are initiated. We will describe the evolution of
the polarization brightness (pB) for the resulting configurations, as
well as the propagation of the ejected plasmoids in the solar wind, and
we will compare the effects of differential rotation with other possible
initiation mechanisms, such as the emergence of new magnetic flux.
Title: Coronal Loop Formation Resulting from Photospheric Convection
Authors: van Hoven, G.; Mok, Y.; Mikic, Z.
Bibcode: 1995ApJ...440L.105V
Altcode:
We have demonstrated the dynamic formation of coronal magnetic loops
in three dimensions as a result of horizontal vortex-like convection
on the photosphere. Localized plasma motions twist bipolar magnetic
field lines which are tied to the dense photosphere by high electrical
conductivity. The twists propagate into the corona along the field
and create a narrow quasi-toroidal region where the field lines
interwind. At the same time, this tubeline region rises in altitude,
expands in cross section, and distorts into a slight S shape before
settling into an equilibrium state. The MHD stability of such
line-tied magnetic loop structures is directly exhibited by this
dynamic simulation.
Title: Disruption of a Helmet Streamer by Photospheric Shear
Authors: Linker, Jon A.; Mikic, Zoran
Bibcode: 1995ApJ...438L..45L
Altcode:
Helmet streamers on the Sun have been observed to be the site of coronal
mass ejections, dynamic events that eject coronal plasma and magnetic
fields into the solar wind. We develop a two-dimensional (azimuthally
symmetric) helmet streamer configuration by computing solutions of the
time-dependent magnetohydrodynamic (MHD) equations, and we investigate
the evolution of the configuration when photospheric shearing motions
are imposed. We find that the configuration disrupts when a critical
shear is exceeded, ejecting a plasmoid into the solar wind. The
results are similar to the case of a sheared dipole magnetic field in a
hydrostatic atmosphere (Mikic & Linker 1994). However, the presence
of the outflowing solar wind makes the disruption significantly more
energetic when a helmet streamer is sheared. Our resutls suggest that
shearing of helmet streamers may initiate coronal mass ejections.
Title: Modeling coronal evolution
Authors: Linker, J. A.; Mikic, Zoran; Schack, Dalton D.
Bibcode: 1994ESASP.373..249L
Altcode: 1994soho....3..249L
No abstract at ADS
Title: Disruption of Coronal Magnetic Field Arcades
Authors: Mikic, Zoran; Linker, Jon A.
Bibcode: 1994ApJ...430..898M
Altcode:
The ideal and resistive properties of isolated large-scale coronal
magnetic arcades are studied using axisymmetric solutions of the
time-dependent magnetohydrodynamic (MHD) equations in spherical
geometry. We examine how flares and coronal mass ejections may be
initiated by sudden disruptions of the magnetic field. The evolution
of coronal arcades in response to applied shearing photospheric
flows indicates that disruptive behavior can occur beyond a
critical shear. The disruption can be traced to ideal MHD magnetic
nonequilibrium. The magnetic field expands outward in a process that
opens the field lines and produces a tangential discontinuity in the
magnetic field. In the presence of plasma resistivity, the resulting
current sheet is the site of rapid reconnection, leading to an
impulsive release of magnetic energy, fast flows, and the ejection of
a plasmoid. We relate these results to previous studies of force-free
fields and to the properties of the open-field configuration. We show
that the field lines in an arcade are forced open when the magnetic
energy approaches (but is still below) the open-field energy, creating
a partially open field in which most of the field lines extend away
from the solar surface. Preliminary application of this model to helmet
streamers indicates that it is relevant to the initiation of coronal
mass ejections.
Title: Thickness Variations along Coronal Loops Inferred from Vector
Magnetograph Data
Authors: McClymont, A. N.; Mikic, Z.
Bibcode: 1994ApJ...422..899M
Altcode:
It has been noted for many years that images of active region coronal
loops seen in soft X-rays or extreme ultraviolet emission suggest a
pipe-like appearance. Recently Klimchuk et el. have quantified this
characteristic, finding for several loops observed by the Soft X-Ray
Telescope on the Yohkoh spacecraft a thickness variation along their
lengths of only 10%-20%. We demonstrate here that this observation
is consistent with the characteristics of current-carrying field
lines in a highly sheared active region. Vector magnetogram data
on NOAA active region 5747, taken with the Stokes Polarimeter at
Mees Solar Observatory on 1989 October 20, provided photospheric
boundary conditions from which a force-free coronal magnetic field was
computed. By tracing field lines, we show that magnetic loops which
are highly sheared do not expand rapidly in height, as they would in
a potential field. In addition, the expanding sections close to the
footpoints of current-carrying twisted loops tend to be more vertical
than in a potential field, so that when seen projected against the
solar disk, the loops appear to terminate more abruptly. Consequently,
current-carrying loops exhibit a near-uniform cross section with
thickness variations of order 30% along their lengths.
Title: Current Filaments Induced in a Resistive Corona by Continuous
Footpoint Motions
Authors: Schnack, Dalton D.; Mikic, Zoran
Bibcode: 1994ASPC...68..180S
Altcode: 1994sare.conf..180S
No abstract at ADS
Title: Deducing Coronal Magnetic Fields from Vector Magnetograms
Authors: Mikic, Zoran; McClymont, Alexander N.
Bibcode: 1994ASPC...68..225M
Altcode: 1994sare.conf..225M
No abstract at ADS
Title: Evolution and Disruption of Magnetic Arcades
Authors: Linker, Jon A.; Mikic, Zoran
Bibcode: 1994ASPC...68..251L
Altcode: 1994sare.conf..251L
No abstract at ADS
Title: The Generation of Solar Magnetic Activity
Authors: van Hoven, Gerard; Schnack, D. D.; Mikic, Z.; Linker, J. A.
Bibcode: 1994ASPC...68..211V
Altcode: 1994sare.conf..211V
No abstract at ADS
Title: Determination of coronal magnetic fields from vector
magnetograms
Authors: Mikic, Zoran
Bibcode: 1993saic.rept.....M
Altcode:
This report covers technical progress during the second year of
the contract entitled 'Determination of Coronal Magnetic Fields from
Vector Magnetograms,' NASW-4728, between NASA and Science Applications
International Corporation, and covers the period January 1, 1993 to
December 31, 1993. Under this contract SAIC has conducted research into
the determination of coronal magnetic fields from vector magnetograms,
including the development and application of algorithms to determine
force-free coronal fields above selected observations of active
regions. The contract began on June 30, 1992 and has a completion
date of December 31, 1994. This contract is a continuation of work
started in a previous contract, NASW-4571, which covered the period
November 15, 1990 to December 14, 1991. During this second year we
have concentrated on studying additional active regions and in using
the estimated coronal magnetic fields to compare to coronal features
inferred from observations.
Title: The Coronal Magnetic Field of AR6919 on 1991 November 15
Authors: McClymont, A. N.; Mikic, Z.
Bibcode: 1993BAAS...25.1214M
Altcode:
No abstract at ADS
Title: Eruption of Large-Scale Coronal Magnetic Fields
Authors: Linker, J. A.; Mikic, Z.
Bibcode: 1993BAAS...25.1204L
Altcode:
No abstract at ADS
Title: Realistic Three-Dimensional Models of Active Region Magnetic
Fields
Authors: Mikic, Z.
Bibcode: 1993BAAS...25Q1218M
Altcode:
No abstract at ADS
Title: Current Filaments Induced in a Resistive Corona by Continuous
Footpoint Motions
Authors: Schnack, D. D.; Mikic, Z.
Bibcode: 1993BAAS...25Q1212S
Altcode:
No abstract at ADS
Title: Determination of coronal magnetic fields from vector
magnetograms
Authors: Mikic, Zoran
Bibcode: 1992saic.reptQ....M
Altcode:
The determination of coronal magnetic fields from vector magnetograms,
including the development and application of algorithms to determine
force-free coronal fields above selected observations of active
regions is studied. Two additional active regions were selected and
analyzed. The restriction of periodicity in the 3-D code which is used
to determine the coronal field was removed giving the new code variable
mesh spacing and is thus able to provide a more realistic description
of coronal fields. The NOAA active region AR5747 of 20 Oct. 1989 was
studied. A brief account of progress during the research performed
is reported.
Title: Coronal Heating Induced by Footpoint Motions
Authors: Schnack, D. D.; Mikić, Z.
Bibcode: 1992AAS...180.0509S
Altcode: 1992BAAS...24..734S
No abstract at ADS
Title: Properties of Coronal Magnetic Fields Calculated from Vector
Magnetograms
Authors: Mikić, Z.; McClymont, A. N.
Bibcode: 1992AAS...180.1204M
Altcode: 1992BAAS...24..748M
No abstract at ADS
Title: Evolution of Large-Scale Coronal Arcades
Authors: Linker, J. A.; Mikić, Z.
Bibcode: 1992AAS...180.0504L
Altcode: 1992BAAS...24..733L
No abstract at ADS
Title: Determination of the coronal magnetic field from vector
magnetograph data
Authors: Mikic, Zoran
Bibcode: 1991saic.rept.....M
Altcode:
A new algorithm was developed, tested, and applied to determine coronal
magnetic fields above solar active regions. The coronal field above NOAA
active region AR5747 was successfully estimated on 20 Oct. 1989 from
data taken at the Mees Solar Observatory of the Univ. of Hawaii. It
was shown that observational data can be used to obtain realistic
estimates of coronal magnetic fields. The model has significantly
extended the realism with which the coronal magnetic field can be
inferred from observations. The understanding of coronal phenomena
will be greatly advanced by a reliable technique, such as the one
presented, for deducing the detailed spatial structure of the coronal
field. The payoff from major current and proposed NASA observational
efforts is heavily dependent on the success with which the coronal
field can be inferred from vector magnetograms. In particular, the
present inability to reliably obtain the coronal field has been a
major obstacle to the theoretical advancement of solar flare theory
and prediction. The results have shown that the evolutional algorithm
can be used to estimate coronal magnetic fields.
Title: Three-Dimensional Stability of a Model Sunspot Field
Authors: Schnack, D. D.; Mikić, Z.
Bibcode: 1991BAAS...23R1036S
Altcode:
No abstract at ADS
Title: Calculation of Force-Free Coronal Magnetic Fields from Vector
Magnetograms
Authors: Mikić, Z.
Bibcode: 1991BAAS...23.1032M
Altcode:
No abstract at ADS
Title: Dynamical Evolution of Twisted Magnetic Flux
Tubes. I. Equilibrium and Linear Stability
Authors: Mikic, Zoran; Schnack, Dalton D.; van Hoven, Gerard
Bibcode: 1990ApJ...361..690M
Altcode:
The three-dimensional dynamical evolution of twisted magnetic flux tubes
is studied using a time-dependent magnetohydrodynamic (MHD) model. The
flux tubes are intended to model solar coronal loops, and include
the stabilizing effect of photospheric line tying. The model permits
the complete evolution of flux tubes to be followed self-consistently,
including the formation, equilibrium, linear instability, and nonlinear
behavior. Starting from an initial uniform background magnetic field,
a twisted flux tube is created by the application of slow, localized
photospheric vortex flows. The flux tube evolves quasi-statically
through sequences of equilibria with increasing twist, until it
becomes linearly unstable to an ideal MHD kink mode. In this paper,
the equilibrium properties and the linear stability behavior are
discussed. The application of the method to the uniform-twist,
Gold-Hoyle field confirms the previous stability threshold for kink
instability and provides estimates of the resulting growth rate.
Title: Magnetohydrodynamic modeling of the solar corona
Authors: Mikic, Zoran
Bibcode: 1990PhFlB...2.1450M
Altcode:
The ideal and resistive magnetohydrodynamic (MHD) model is used to
examine the dynamics and structure of the solar corona. When the coronal
magnetic field is deformed by photospheric flow it can evolve to states
that become unstable to ideal MHD modes. The nonlinear evolution of
these instabilities can lead to the generation of current sheets, field
line reconnection, and energy release. The disruption of an arcade
field and the kinking of coronal loops is described. The braiding
of the large-scale coronal field by convective photospheric motions
develops fine-scale structure in the magnetic field and leads to the
development of intense current filaments. The resistive dissipation
of these currents can provide an efficient coronal heating mechanism.
Title: Magnetohydrodynamic simulation of coronal magnetic fields
Authors: Schnack, D. D.; Mikić, Z.; Barnes, D. C.; van Hoven, G.
Bibcode: 1990CoPhC..59...21S
Altcode:
The application of supercomputers and advanced numerical techniques
to problems of coronal structure and dynamics is described. Numerical
methods appropriate for the long time scale simulation of nonlinear
magnetohydrodynamic systems are discussed. Three specific examples of
the application of these techniques to the solar corona are given. These
are magnetic energy storage and conversion, a model for steady coronal
heating, and calculation of stable force-free equilibria from given
boundary data, such as that obtained with a vector magnetograph. It is
suggested that the continued application of these methods will result
in substantial advances in the understanding of coronal dynamics
and structure.
Title: Creation of Current Filaments in the Solar Corona
Authors: Mikic, Z.; Schnack, D. D.; van Hoven, G.
Bibcode: 1989ApJ...338.1148M
Altcode:
It has been suggested that the solar corona is heated by the dissipation
of electric currents. The low value of the resistivity requires the
magnetic field to have structure at very small length scales if this
mechanism is to work. In this paper it is demonstrated that the coronal
magnetic field acquires small-scale structure through the braiding
produced by smooth, randomly phased, photospheric flows. The current
density develops a filamentary structure and grows exponentially in
time. Nonlinear processes in the ideal magnetohydrodynamic equations
produce a cascade effect, in which the structure introduced by the
flow at large length scales is transferred to smaller scales. If this
process continues down to the resistive dissipation length scale,
it would provide an effective mechanism for coronal heating.
Title: Calculation of the Coronal Magnetic Field from Vector
Magnetograph Data
Authors: Mikic, Z.; Barnes, D. C.; Schnack, D. D.
Bibcode: 1989BAAS...21..855M
Altcode:
No abstract at ADS
Title: Generation of fine-scale structure in the solar magnetic field.
Authors: Mikič, Z.; Schnack, D. D.
Bibcode: 1989tndm.conf..113M
Altcode:
The authors investigate the structure of the magnetic field by solving
the 3D MHD equations numerically. They describe two realizations of
footpoint motions which are characteristic of photospheric convection:
spatially smooth flows with a random time behavior (characteristic of
granulation flows), and coherent vortex flows which produce twisted
flux tubes.
Title: Dynamical Evolution of a Solar Coronal Magnetic Field Arcade
Authors: Mikic, Z.; Barnes, D. C.; Schnack, D. D.
Bibcode: 1988ApJ...328..830M
Altcode:
Calculations of the long-term dynamical evolution of a solar coronal
magnetic field arcade which is subjected to shearing photospheric
flows are presented. The evolution is obtained by numerical solution
of a subset of the resistive magnetohydrodynamic equations. For a
simplified model of the bipolar magnetic field observed in the solar
corona, it is found that photospheric flow produces a slow evolution
of the magnetic field, with a buildup of magnetic energy. For certain
photospheric shear profiles, the field configuration produced is
linearly unstable to an ideal magnetohydrodynamic mode when the shear
exceeds a critical value. The nonlinear evolution of this instability
shows the spontaneous formation of current sheets. Reconnection of
the magnetic field produces a rapid release of magnetic energy. The
major fraction of the energy is dissipated resistively, while a small
fraction is converted into kinetic energy of an ejected plasmoid. The
relevance of these results to two-ribbon flares is discussed.
Title: Creation of Current Filaments in the Solar Corona
Authors: Mikic, Z.; Schnack, D. D.; van Hoven, G.
Bibcode: 1988BAAS...20..682M
Altcode:
No abstract at ADS
Title: Ideal MHD stability of solar coronal arcades.
Authors: Barnes, D. C.; Mikic, Z.; Schnack, D. D.
Bibcode: 1987BAAS...19.1121B
Altcode:
No abstract at ADS
Title: Ideal MHD Stability of Solar Coronal Arcades
Authors: Bames, D. C.; Mikic, Z.; Schnack, D. D.
Bibcode: 1987BAAS...19R1121B
Altcode:
No abstract at ADS
Title: Nonlinear Dynamics of the Force-Free Coronal Arcade Field
Due to Shear Flow
Authors: Bekki, N.; Tajima, T.; Mikic, Z.; Barnes, D. C.; Schnack,
D. D.
Bibcode: 1987BAAS...19Q1121B
Altcode:
No abstract at ADS
Title: Dynamical Evolution of a Solar Coronal Arcade
Authors: Mikic, Z.; Barnes, D. C.; Schnack, D. D.; Tajima, T.;
Bekki, N.
Bibcode: 1987BAAS...19..922M
Altcode:
No abstract at ADS
Title: Energy Buildup and Explosive Release in a Sheared Coronal
Arcade Plasma*
Authors: Tajima, T.; Bekki, N.; Mikic, Z.; Barnes, D.; Schnack, D.
Bibcode: 1987BAAS...19..922T
Altcode:
No abstract at ADS
Title: Magnetic Energy Storage and Conversion in Coronal Arcades
and Loops
Authors: Mikic, Z.; Barnes, D. C.; Schnack, D. D.; Steinolfson, R. S.;
Tajima, T.; Zaidman, E.
Bibcode: 1986BAAS...18Q1042M
Altcode:
No abstract at ADS
Title: Resistive Evolution of Coronal Arcades
Authors: Mikic, Z.; Barnes, D. C.; Schnack, D. D.; Steinolfson, R. S.;
Tajima, T.
Bibcode: 1986BAAS...18..676M
Altcode:
No abstract at ADS
Title: Dynamic Evolution of Coronal Magnetic Fields
Authors: Tajima, T.; Steinolfson, R. S.; Barnes, D. C.; Mikic, Z.;
Schnack, D. D.
Bibcode: 1986BAAS...18..676T
Altcode:
No abstract at ADS